Added multi-folder support to cptcmap.m, and updated cptcbar.m doc

In order to make it easier to upload a select number of .cpt files
in my public distribution, while still keeping a much larger library
for personal use, I've added support for multiple .cpt folders.

Also, updated the documentation for cptcbar, which was previously
incomplete.
Merge commit 'a25c93093c423bfa6e51669824cdef075d8f72af'