0.7.4: Minor release

CHANGELOG:

New features

export_markers_to_csv in DataStore enables exporting marker tables

Improvements

Improved Docstrings (thanks to @razofz)
Improved vignettes (thanks to @JohanRodhe)
Removed stale shared libs for sgtsne.
Added reset_previous parameter to filter_cells in DataStore
Added reset_cell_filter parameter to ZarrMerge. All cell attributes are now copied into merged Zarr file