If no cells are requested, return a NULL;
by default, throws an error
cells
Subset of cell names
expression
A predicate expression for feature/variable expression,
can evaluate anything that can be pulled by FetchData; please note,
you may need to wrap feature names in backticks (``) if dashes
between numbers are present in the feature name
invert
Invert the selection of cells
idents
A vector of identity classes to keep
slot
Slot to pull feature data for
downsample
Maximum number of cells per identity class, default is
Inf; downsampling will happen after all other operations, including
inverting the cell selection
seed
Random seed for downsampling. If NULL, does not set a seed