sensible output from levels command when run on symmetric CpG counts files

[andrewdavidsmith]

A common scenario is to analyze methylation levels in subsets of the CpG sites (or some other specific local context) relative to some genomic intervals. This would mean that much of the typical output from the levels command contains invalid information -- lots of nan, etc. I wonder if there is a way to either circumvent this messy output, or allow the user to request only certain output among all of what levels can provide.