Allow SlideData to use existing h5path files

[tddough98]

As motivated by #332 and #300, this modifies SlideData to read and update Tiles from an existing h5path file instead of requiring each pipeline run to recreate all tiles from scratch.

This includes #335 as many transforms (e.g. BoxBlur) require np.uint8 data instead of the default float16 saved to h5path files. I was also working off my load-data-in-workers branch because it had significant performance changes for my use cases. Sorry about the branching messiness, hopefully the changes will be clearer as other branches are merged into dev.

This makes breaking changes to the SlideData API, namely replacing generate_tiles with get_tiles and moving the tile parameterization from run to the SlideData constructor.