Improve testing of `sample_weight`

[gbrener]

Per @PeterDSteinberg's comment on PR #181:

"...better test the usage of sample_weight. This has come up now due to low test coverage on its usage."

Deferred until after we address #171 .

[PeterDSteinberg]

Tests should cover the following:

• [ ] sample_weight is passed to fitting methods (what about transform methods?)
• [ ] sample_weight is not passed to prediction methods
• [ ] sample_weight is a 1-D NumPy array that matches X's row count (and y's row count if applicable)
• [ ] sample_weight may be something other than a 1-D NumPy array but with the correct row count (we may defer this bullet point, depending on our progress with data structure flexibility overall when this testing issue is addressed).