Units option on *calabrate

[Andrew]

Currently the calibrate adjusts for the offset then multiplies by the scale factor, which works for a stretched tape with the end missing *calibrate tape 0.05 0.752

But of thinks like mounting a DistoX in a waterproof case the offset will be measured with another device so will not want scaling

Suggestion is to allow the addition of units to the offset if the scale factor should not be applied, this should allow the original format to continue to work while adding functionality.

if not units

real = ( reading - offset) * scale

If units eg *calibrate tape -0.05m 0.752

real = reading * scale - offset