Allow use of several simultaneous tool offsets with G43.2 H

To use tool 1 with the sum of offsets 1 and 10, program:
T1 M6
G43 (or G43 H1: replace any existing offset with T1's)
G43.2 H10 (also add in the T10 offset)

You can sum an arbitrary number of offsets by calling G43.2 more times.
There are no built-in assumptions about which numbers represent geometry
and which are wear, or that you should have only one of each.
Touchy support for mdi G43.2
G43.1 works with all axes, not just XZ now
Documentation for G43.2
A demo config combining cradek's compounf offset (G43.2) and mah's
remapping capability to redefine toolchange to be more similar to many
CNC lathes.
The tool table pocket number is ignored. To access that requires an emccanon
command in the prologue or post-logue python code, and example of which
can be seen in the stdglue.py prepare_epilog() code.

Signed-off-by: andy pugh <andy@bodgesoc.org>
g43.2: verify H-word is present, as per the docs
tests: add a test of G43 & G43.2
Start with a clean var file each time, since we might have rotated
The interpreter doesn't report very many decimal places to the UI
Also test clear/set TLO and summing TLO while rotated