Add AnnData dense matrix read support

[bnprks]

This addresses old issues #17 and #36 by adding support for reading the AnnData dense matrix format.

Changes:

• Add support for reading dense AnnData matrices
• Adjust test files for old AnnData versions to also cover obsm, varm, and dense matrices. Reduce the version numbers covered since v0.7.8 appeared to be identical to v0.7.0
• Adjust how dimnames are recognized from AnnData files (match based on length rather than always assuming var is rows and obs is cols. This helps support other matrix shapes as found in obsm, or varp, etc.)
• Split the 10x and AnnData import code into separate files since they are getting a bit long otherwise

The first commit is the main one to look at for review, the others are moving around code mostly.

[bnprks]

As discussed offline, another to-do item is filtering out explicit zeros while reading from a dense matrix