MOFA: >1TB RAM needed for ATAC

[wlason]

I see that for datasets >100k cells, over 1TB of RAM is needed to run MOFA with default HVF selection.
To lower the memory requirements, subsetting to a lower number of ATAC peaks, such as 25k, solves the issue.

I also asked MOFA developers to give an indication of how much RAM is typically needed: bioFAM/mofapy2#32