Z probe code improved, but still not there.  It now uses hysteresis to
track up the low-precision slope of the IR probe before measuring down
the high-precision slope near 0 mm.  But this means that the probe has to
be set only a fraction of a mm higher than the nozzle itself.  Also (at
the moment for safety) the code thinks Z = 0 occurs when it is actually
0.2mm.  This is easy to fix when the rest is reliable.