New .pgxfreq Interval Frequencies File Type

Changed suffix and upcoming additions...

With the November 2022 update we changed the file suffix to pgxfreq to keep a clean separation between the (usually binned) CNV frequency files and the (usually raw) representation of sample-specific CNVs (and other variants).

The overall format hasn't changed from the previous implementation although we intend to add representation for different CNV levels in future updates.