Update pooch downloader: use CESMDATAROOT variable when available

[andersy005]

Addresses permission issue when local storage path points to a location that we don't have permissions to write to.

Closes #51

[andersy005]

Here's an example of this in action on casper:

In [1]: from pop_tools.grid import grid_defs

In [2]: example = grid_defs['POP_gx1v7']

In [3]: example
Out[3]:
{'lateral_dims': [384, 320],
 'vertical_dims': 60,
 'vert_grid_file': 'gx1v7_vert_grid',
 'horiz_grid_fname': 'inputdata/ocn/pop/gx1v7/grid/horiz_grid_20010402.ieeer8',
 'topography_fname': 'inputdata/ocn/pop/gx1v7/grid/topography_20161215.ieeei4',
 'region_mask_fname': 'inputdata/ocn/pop/gx1v7/grid/region_mask_20151008.ieeei4',
 'type': 'dipole',
 'region_mask_regions': {'Black Sea': -13,
  'Baltic Sea': -12,
  'Red Sea': -5,
  'Southern Ocean': 1,
  'Pacific Ocean': 2,
  'Indian Ocean': 3,
  'Persian Gulf': 4,
  'Atlantic Ocean': 6,
  'Mediterranean Sea': 7,
  'Lab. Sea & Baffin Bay': 8,
  'GIN Seas': 9,
  'Arctic Ocean': 10,
  'Hudson Bay': 11}}

In [4]: from pop_tools.grid import INPUTDATA

In [5]: from pop_tools.datasets import fetch

In [6]: path = fetch(INPUTDATA, fname=example['horiz_grid_fname'])

In [7]: path
Out[7]: '/glade/p/cesmdata/cseg/inputdata/ocn/pop/gx1v7/grid/horiz_grid_20010402.ieeer8'

[matt-long]

@andersy005, thanks for your prompt response to this issue.

One thing I find annoying is the statement when importing pop_tools, that the intputdata directory is not writable. Will this PR remove that message?

If inputdata is not writable, we effectively have to assume that all files are present. I see two potential design responses to this:

1. Explicitly switch-off pooch and make fetch use direct access.
2. Have the option of multiple cache directories in order of preference, if a file is not found in 1, use pooch to download it to 2.

What do you think?