Adaptation: snapTool fragment files

[rootze]

Due to the issues reported in #3, we suspected the error was due to the fragments' format. We are working with other researchers who have access to SnapTool output files to investigate the matter actively.

• Finding a way to reformat snapTool fragment files
  Possible way of reformating fragment files to match cellranger atac count output: stuart-lab/signac#748
• Update the ArchR2Signac code to accept the reformatted SnapTool fragment files
• Update the ArchRtoSignac tutorial