Refactor tests for better error messages

[j-adamczyk]

Changes

Refactoring of tests to use np.testing.array_equal primarily to compare values. It very nicely prints the actual and expected values, in comparison to raw assert.

Previous:

>       assert np.array_equal(X_skfp + 1, X_rdkit)
E       AssertionError

tests/fingerprints/atom_pair.py:20: AssertionError

Current:

>       assert_equal(X_skfp + 1, X_rdkit)
E       AssertionError: 
E       Arrays are not equal
E       
E       Mismatched elements: 204800 / 204800 (100%)
E       Max absolute difference among violations: 1
E       Max relative difference among violations: 1.
E        ACTUAL: array([[1, 1, 1, ..., 1, 1, 1],
E              [2, 2, 2, ..., 1, 1, 1],
E              [1, 1, 1, ..., 1, 1, 1],...
E        DESIRED: array([[0, 0, 0, ..., 0, 0, 0],
E              [1, 1, 1, ..., 0, 0, 0],
E              [0, 0, 0, ..., 0, 0, 0],...

Checklist before requesting a review

• Docstrings added/updated in public functions and classes
• Tests added, reasonable test coverage (at least ~90%, make test-coverage)
• Sphinx docs added/updated and render properly (make docs and see docs/_build/index.html)