Improve tests

[maxrjones]

Description of proposed changes

This PR improves the xbatcher test suite through the following changes:

• Use xarray testing methods on batches for more informative diffs
• Add utility functions for testing the dimensions and shape of batches and the batch generator length. These add a proportionally large amount of code, but are helpful for catching inconsistencies and preventing regressions.
• Add a test for the batch_dims parameter. As expected, the test currently fails due to Batch generation with batch_dims in v0.2.0 is about 10-20times slower #121
• Add docstrings to the tests

To-do:

• Open a few issues based on some inconsistencies I noticed when working on this PR

Addresses #83

[maxrjones]

Noting that the failing tests (detailed below) are expected due to the bug reported in #121. My opinion is that it is best to merge this PR with the failing tests and fix the bug in a separate PR.

FAILED xbatcher/tests/test_generators.py::test_batch_3d_1d_input_batch_dims[True] - AssertionError: {'sample': 200, 'x_input': 5, 'y_input': 10} != {'y_input': 10, 'x_input': 5, 'sample': 1000}
- {'sample': 200, 'x_input': 5, 'y_input': 10}
?            ^

+ {'sample': 1000, 'x_input': 5, 'y_input': 10}
?            ^^
 : Dimension names and/or lengths differ
FAILED xbatcher/tests/test_generators.py::test_batch_3d_1d_input_batch_dims[False] - AssertionError: {'time': 2, 'x': 5, 'y': 10} != {'time': 10, 'y': 10, 'x': 5}
- {'time': 2, 'x': 5, 'y': 10}
?          ^

+ {'time': 10, 'x': 5, 'y': 10}

@jhamman would you be able to provide a review for this PR?