Add units of measurement

[Chuck]

As in "pounds", "cm^{3", etc. Add these at least at the library level, but potentially at the language level.}

Please feel free to add comments to this ticket including interesting links, proposed ways to do implement it, syntax, etc.

[Chuck]

One proposal is at:

•  http://cobra-language.com/forums/viewtopic.php?f=4&t=287#p1720