ngcgui-gcmc: always pass floating point

avoid subtle problems with gcmc using integers
ngcgui-gcmc programmable involute-gear example

new: involute-gear.gcmc adapted for ngcgui from B. Stultien's example
new: ensure_mode.gcmd include file to resolve potential conflict
     with preamble and gcmc code (gcmc wins)

with updates:
1) popup msg error if entry box for gcmc not a number
2) allow comment after tag when tagging a  gcmc option
3) handle gcmc_include_path for new custom tab additions
4) incorporate ensure_mode in .gcmc examples
5) update ngcgui.txt for ensure_mode