MNT : refactored CrossSection2DView

 - pulled single-frame handling out from data-management super-classes
 - re-did how limit functions work, need to fix messenger class
MNT : converted limit functions -> limit factories

 - generate and return closures over the limit arguments
MNT : updated crosssection2D messenger

 - works with re-factored underlying class again
 - removed independent signals/slots for setting the color limits
DOC : improved documentation
DOC : added comments

Clarified what `vlim` does
Merge pull request #19 from tacaswell/refactor_xsection2D

Refactor xsection 2d