ScPreprocess: Add sparse support

[pavlin-policar]

Issue

single_cell.preprocess.scpreprocess does not support sparse data.

Description of changes

Add sparse support to preprocessors. Does not include single_cell.preprocess.scpreprocess.Standardize, as some thought will have to be put into how to handle this elegantly.

Includes

• [X] Code changes
• [X] Tests
• [ ] Documentation