add name to peakSet / PeakMatrix

[rcorces]

Originally, peak sets do not have a name column. This means that when getFeatures() is called, it returns NULL because no name column is present. This makes it impossible to use plotEmbedding() or addModuleScores() with peaks. To address this, I've added a line that automatically names peaks as seqname_peakIndex such as "chr1_peak1".

I dont think that this will break anything downstream but would be good to have a second opinion

addressing https://github.com/GreenleafLab/ArchR/discussions/1421 and https://github.com/GreenleafLab/ArchR/issues/308#issuecomment-792338095