Index homing Heidenhain ls 403/exe602

Truly grateful for all support here!

I figured it could be tested with a normal rotary encoder by setting the "home" detection 
index interval to the CPR of the encoder, so 2 index detection's with the proper delta
would arm LinuxCNC's index search.

I guess a fancier scheme would use it as an absolute scale ( generate the correct absolute
position after any  two index detection's  and a table lookup ) but I think that would need
a new homing routine rather than just a hal component.  

I am still looking into this, I did make a small change to the encoder driver to simplify
the index spacing detection. (I added the option to not zero the position on index detection)
This should simplify the distance coded index detection hal component, otherwise I would
have needed to undo the encoder position zeroing in the hal component, and that was pretty ugly.