Porting Trotec lasercutter Fab Module to mods

Porting Trotec fabmodule to mods, using Epilog mod/fabmodule conversion as a model. A couple of questions regarding variable names:

• speed vs velocity -- in the Epilog mod and fab module, speed is speed; for Trotec it's called velocity. It's also scaled by a factor, though I believe this is just to adjust to different Trotec models.
• rate vs frequency -- same issue: Epilog has a rate, Trotec has a frequency.

Are these differences intentional? At the moment the variable names in the Trotec mod correspond to the fabmodule (rather than the Epilog mod).

The new module is on the branch trotec-port