Clarify link between genome and bedfile

[callunity]

Is your feature request related to a problem? Please describe.
If the user chooses genome == 'hg38', the bedfile is automatically set to the included hg38_cytoBand.txt file which will silently overwrite any args.bedfile value the user has chosen.

Describe the solution you'd like
Only overwrite args.bedfile if this value is not already specified by the user.

Describe alternatives you've considered
Update the documentation to make very clear that if hg38 is selected, it will use the inbuilt bedfile.