Z probing now working.  The probe signal connects to AD7 which (confusingly)
is A0 on the Arduino.  The signal seems quite noisy.  I have put a 10uF
capacitor across its supply.  Maybe add an 0.1uF across its signal?
Two values need to be set in Platform.h:

If v is the value from the A->D on A0, then:

z = v*Z_PROBE_GRADIENT + Z_PROBE_CONSTANT (mm)

Z_PROBE_GRADIENT is dependent on the IR proximity device (though they may
be consistent enough all to have the same value) and Z_PROBE_CONSTANT depends
on how high it is mounted relative to the nozzle.

To generate these values, uncomment the bits of code in and around Platform::Spin()
zero the Z axis by eye, run the program, and raise Z by 0.1mm increments.  Put
the raw A->D values in the spreadsheet in the Data directory, and it will
give you the values of Z_PROBE_GRADIENT and Z_PROBE_CONSTANT.