min_cells argument to load_and_normalize()

[jjschirle]

Adding min_cells argument to load_and_normalize(). This filters out genes that are not uniquely expressed in at least (<) min_cells cells.

Note: This is different than gene_threshold, which filters genes based on total raw counts across cells.

Intended as a quick patch, future development should integrate this functionality within an object-oriented ecosystem. One could envision a future variant of load_and_normalize() with the following functionality:

Parameter/concept	Filters	Criterion
min_cell_counts	cells	total counts in the cell
min_genes	cells	number of genes detected in the cell
gene_threshold	genes	total expression/counts across cells
min_cells	genes	number of cells where gene is detected