unit_testing: test file names

[jpolton]

test files used in unit_testing should include parent class in name.

This started with test_index_class_load_datasets.py which grouped all the reading methods for index objects.
However it makes more sense to group the test files by child object, complete with their methods, rather than a horizontal slice across each child object pulling out e.g. all the equivalent read methods.

Nevertheless there is also some elegance in having all the indexed class load methods together. This helps identify / group all the class objects that are children of indexed.

So, propose better filenaming to help clarify which is the parent object.

E.g.
test_isobath_contour_methods.py --> test_gridded_isobath_contour_methods.py
test_altimetry_methods.py --> test_indexed_altimetry_methods.py

Also test_index_class_load_datasets.py isn't actually tested but is a useful place to keep stub development for indexed children objects. This should be purge to retain only the undeveloped/untested methods (see also notes in file header):

• EGO Gliders
• ARGO floats
• OceanParcels data
  and perhaps the file named to WIP_test_index_class_load_datasets.py