Add select from Example structures widget back (#215)

Add the `StructureExamplesWidget` back to structure manager widget
with more examples. 

Add two example structures `Au.cif` which is not a primitive cell and `Co.cif` which has spin-polarization turned on.

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aiidalab_qe/structures.py

@@ -27,6 +27,17 @@
 </strong></p>
 </div>"""
 
+# The Examples list of (name, file) tuple curretly passed to
+# StructureExamplesWidget.
+Examples = [
+    ("Silicon (diamond)", "miscellaneous/structures/Si.xyz"),
+    ("Silicon oxide", "miscellaneous/structures/SiO2.xyz"),
+    ("Diamond", "miscellaneous/structures/diamond.cif"),
+    ("Gallium arsenide", "miscellaneous/structures/GaAs.xyz"),
+    ("Gold (fcc)", "miscellaneous/structures/Au.cif"),
+    ("Cobalt (hcp)", "miscellaneous/structures/Co.cif"),
+]
+
 
 class StructureSelectionStep(ipw.VBox, WizardAppWidgetStep):
     """Integrated widget for the selection of structures from different sources."""

miscellaneous/structures/Au.cif

@@ -0,0 +1,36 @@
+#======================================================================
+
+# CRYSTAL DATA
+
+#----------------------------------------------------------------------
+
+data_VESTA_phase_1
+
+
+_pd_phase_name                         'Au                                    '
+_cell_length_a                         4.17410
+_cell_length_b                         4.17410
+_cell_length_c                         4.17410
+_cell_angle_alpha                      90
+_cell_angle_beta                       90
+_cell_angle_gamma                      90
+_symmetry_space_group_name_H-M         'P 1'
+_symmetry_Int_Tables_number            1
+
+loop_
+_symmetry_equiv_pos_as_xyz
+   'x, y, z'
+
+loop_
+   _atom_site_label
+   _atom_site_occupancy
+   _atom_site_fract_x
+   _atom_site_fract_y
+   _atom_site_fract_z
+   _atom_site_thermal_displace_type
+   _atom_site_B_iso_or_equiv
+   _atom_site_type_symbol
+   Au1      1.0    0            0            0           Biso  1.000  Au
+   Au2      1.0    0            0.50000      0.50000     Biso  1.000  Au
+   Au3      1.0    0.50000      0            0.50000     Biso  1.000  Au
+   Au4      1.0    0.50000      0.50000      0           Biso  1.000  Au

miscellaneous/structures/Co.cif

@@ -0,0 +1,34 @@
+#======================================================================
+
+# CRYSTAL DATA
+
+#----------------------------------------------------------------------
+
+data_VESTA_phase_1
+
+
+_pd_phase_name                         'Co                                    '
+_cell_length_a                         2.49680
+_cell_length_b                         2.49680
+_cell_length_c                         4.03081
+_cell_angle_alpha                      90
+_cell_angle_beta                       90
+_cell_angle_gamma                      60.00000
+_symmetry_space_group_name_H-M         'P 1'
+_symmetry_Int_Tables_number            1
+
+loop_
+_symmetry_equiv_pos_as_xyz
+   'x, y, z'
+
+loop_
+   _atom_site_label
+   _atom_site_occupancy
+   _atom_site_fract_x
+   _atom_site_fract_y
+   _atom_site_fract_z
+   _atom_site_thermal_displace_type
+   _atom_site_B_iso_or_equiv
+   _atom_site_type_symbol
+   Co1      1.0    0            0            0           Biso  1.000  Co
+   Co2      1.0    0.33333      0.33333      0.50000     Biso  1.000  Co

qe.ipynb

@@ -41,7 +41,7 @@
     "\n",
     "from aiida.orm import StructureData\n",
     "from aiida.orm import load_node\n",
-    "from aiidalab_widgets_base import OptimadeQueryWidget\n",
+    "from aiidalab_widgets_base import OptimadeQueryWidget, StructureExamplesWidget\n",
     "from aiidalab_widgets_base import StructureBrowserWidget, StructureManagerWidget, StructureUploadWidget\n",
     "from aiidalab_widgets_base import WizardAppWidget, WizardAppWidgetStep\n",
     "from aiidalab_widgets_base.bug_report import install_create_github_issue_exception_handler\n",
@@ -51,7 +51,7 @@
     "from aiidalab_qe.steps import ConfigureQeAppWorkChainStep\n",
     "from aiidalab_qe.steps import SubmitQeAppWorkChainStep\n",
     "from aiidalab_qe.steps import ViewQeAppWorkChainStatusAndResultsStep\n",
-    "from aiidalab_qe.structures import StructureSelectionStep\n",
+    "from aiidalab_qe.structures import StructureSelectionStep, Examples\n",
     "from aiidalab_qe.version import __version__\n",
     "\n",
     "\n",
@@ -64,6 +64,7 @@
     "        StructureUploadWidget(title=\"Upload file\"),\n",
     "        OptimadeQueryWidget(embedded=False),\n",
     "        StructureBrowserWidget(title=\"AiiDA database\"),\n",
+    "        StructureExamplesWidget(title=\"From Examples\", examples=Examples),\n",
     "    ],\n",
     "    node_class='StructureData',\n",
     "    storable=False,\n",