feat: add option to test higher pc_mito tail

[RHReynolds]

For QC of single-nucleus RNA-sequencing, only need to look for outliers in the higher tail of mitochondrial counts, as good quality nuclei should not contain any mitochondrial transcripts. Thus, added an option to allow users to specify this in annotate_sce(). Resolves https://github.com/combiz/scFlow/issues/187