Moving CIGAR and positions to common.avdl.

All these are types that would be useful to have for the graph reference/variation stuff, so they would be helpful to have in common.avdl.

Also changing GAMappingPosition to just GAPosition, and commenting everything nicely for avrodoc.
Importing only common.avdl in referenceStructure.avdl now that the CIGAR types are there.
Merge pull request #81 from adamnovak/expandCommon

Moving CIGAR and positions to common.avdl.