[Therion] Survex (loop closure) related fixes

[Tarquin Wilton-Jones]

Hi folks,

Stacho fixed these "when using Survex loop closure" bugs in yesterday's
development release:

https://github.com/therion/therion/issues/269 Therion miscalculates
fixes in multiple centreline blocks when using Survex loop closure

https://github.com/therion/therion/issues/270 Therion cannot have
exactly 2 fixes for a station when using Survex loop closure

And this Survex compatibility bug:

https://github.com/therion/therion/issues/263 Scale value should default
to 1 when omitted from "calibrate" commands

I have checked and verified the fixes, and my surveys still look right.
But could someone with a bigger dataset check that it didn't break
anything else when using Survex loop closures. That particular fix looks
like the kind that could create regressions. It only improved things for
me though :)

Yes, your cave entrance locations will change if you ever ran into #269
without realising - they are supposed to. And yes, you might find some
parts of the cave change size if you ever hit the calibrate statement bug.

Cheers,

Tarquin