[Therion] Scope of calibrate directive

[Tarquin Wilton-Jones]

> I have only ever used calibrate at the start of a relevant centreline, so
> have not tested Tarquin's suggestion, but I'm sure you have Tarquin :)

Indeed. I used it while surveying a sump, pointing the laser into it to 
measure the depth, calibrating the length for the refractive index of 
the water to get the actual depth (and yes, I did all the maths to work 
out how much was in water and how much was in air, because of course I 
did), rather than what the Disto thought it was.

I interleaved the data with the relevant:
1 2 3.4 56 -7.89
calibrate tape 0 0.96
2 3 7.8 90.1 -78.9
calibrate tape 0 1
2 4 1.2 34 5.6

Part way through the data, one calibrate command sets it to a new value, 
and then later another sets it back to defaults ("0 1" in the case of tape).

Other centreline blocks already reset to default without needing to be 
explicitly stated, so they are not affected.

(And for those who care, yes, of course the refraction also altered the 
clinometer readings, but I was only using them to splay the walls and 
measure the deep spots, so I kept it mostly vertical, and pretended not 
to care about the imperfections!)

Sent from my phone earlier, so I couldn't check the specifics.