CoreXY: first draft.
CoreXY: get math for delta_um[] right.

Explanation at http://www.corexy.com/theory.html
CoreXY: get math for steps right, too.

Also add some debugging code.
CoreXY: also calculate motor movement direction correctly.

CoreXY should be complete now.
core xy functionality

this is a working copy of the corexy code
Merge remote-tracking branch 'origin/corexy' into corexy