Cleanup after PR review of momentum 3B-NMD kernel

Closes #19.

[ ] Bug fix (non-breaking change which fixes an unintended or erroneous behavior)
[ ] New features (non-breaking change that adds functionality)
[ ] Breaking change (fix or feature that changes how existing functionality works, other than correcting errors)
[x] Code improvement (no new functionality, but improving project structure)
[x] Documentation only (changes that improve or expand instructions to users, without changing program behavior)

Motivation and Context

Collection of small improvements for the changes introduced with #13: docstrings, typos and additional tests.

This PR introduces the following changes:

Typos and docstrings
- "Utility functions for base model-free kernels. ~~Most likely will not be expanded, even as other naive kernels are added.~~"
- typo: docstring for apply_momentum should be post_momentum_X = apply_momentum(X, X_previous, beta=0.0) instead of beta=1.0
- model_free_util.py
Altered test_instantiate_kernel_honors_strategy_selection to also test Momentum3BlockModelFreeKernel
- test_factory_util.py
Added two test cases for apply_momentum:
- Case 1: beta=1.0, which doubles the 'step' taken from the previous to the current iteration
- Case 2: beta=0.0, which just returns the current matrix without altering it
- test_model_free_util.py
Added two test cases for kernel instantiation of Momentum3BlockModelFreeKernel:
- Case 1: if candidate factor W0is given but H0 is not
- Case 2: if candidate factor W0is not given but H0 is
- In both cases W0 and H0 will be reinstantiated using SVD on low_rank_candidate_L
- Added a fixture and a test for each case
- test_momentum_three_block_model_free_kernel.py

Individual commits follow the order in #19 - one commit for one list item.

[x] My code follows the project code style
- [x] I've run the automatic formatters and made any needed changes
- [x] I've ensured that my new code is explained in docstrings/comments
[ ] My submission requires a change to the documentation
- [ ] I've updated the documentation to reflect the changes
[x] I've added unit tests that cover the visible behavior of the changes
[ ] I've added or modified integration tests that ensure the whole system works on a practical problem after my changes