Fixed aspect ratio for ContourPlot
Importing operation into extension to register channel operations
Added dimension comparisons to utils and improved method ordering
Made DimensionTest and NdIndexableMappingTest into ViewTestCases
Implemented partial fixes to the dim_dict and add_dimension tests

Tuple comparison is failing as the ids of the dimension objects is
changing unexpectedly even though their contents match correctly.
Removed broken test_idxmapping_data_type_check test
Fixed AttrTree doctest in collector.py