Moving the group_by_location function to the stack-based example so t…

…here is no confusion about the internal use of stacks in the v2 codebase.

examples/band_stats.py

@@ -27,14 +27,22 @@
 
 from affine import Affine
 
-from datacube.api import make_storage_unit, group_storage_units_by_location
+from datacube.api import make_storage_unit
 from datacube.index import index_connect
 from datacube.storage.access.core import StorageUnitStack, StorageUnitVariableProxy
 from datacube.ui import parse_expressions
 
 from datacube.storage.storage import fuse_sources, _dataset_time, DatasetSource, RESAMPLING
 
 
+def group_storage_units_by_location(sus):
+    dims = ('longitude', 'latitude')
+    stacks = {}
+    for su in sus:
+        stacks.setdefault(tuple(su.coordinates[dim].begin for dim in dims), []).append(su)
+    return stacks
+
+
 def ndv_to_nan(a, ndv=-999):
     a = a.astype(numpy.float32)
     a[a == ndv] = numpy.nan