Check for MACS2 before splitting file

[AmelZulji]

When calling peaks using:

peaks <- CallPeaks(
  object = seu,
  group.by = "cell_type",
  macs2.path = "wrong_path"
)

splitting fragments will start before checking whether specified macs2.path points to existing instance of MACS2. It would be nice to first check for MACS2.