Add function for calculating niches

src/squidpy/gr/__init__.py

@@ -5,6 +5,7 @@
 from squidpy.gr._build import mask_graph, spatial_neighbors
 from squidpy.gr._ligrec import ligrec
 from squidpy.gr._nhood import centrality_scores, interaction_matrix, nhood_enrichment
+from squidpy.gr._niche import calculate_niche
 from squidpy.gr._ppatterns import co_occurrence, spatial_autocorr
 from squidpy.gr._ripley import ripley
 from squidpy.gr._sepal import sepal