fix infinite loop in commonPrefixMulti

when asking for the common prefix of 0 strings
add unpadded start/stop functions to vcf entry
add CoordinateView abstraction

This lets us specify a type to extract chr/start/stop from objects. The
first use is to have a version that extracts raw positions from a vcf
entry and another that extracts unpadded coords.
update GroupOverlapping to use CoordinateView
remove execute method from GroupOverlapping

now it gets used via streampump