[Therion] 2 fixes for a point with standard deviation

[Tarquin Wilton-Jones]

Therion really doesn't seem to like using Survex for fixed points :(

When you have exactly 2 fixes for a station, Therion 5.5.1 (with Survex
1.2.42) gives up when processing survey data, and gives no reason why.
https://github.com/therion/therion/issues/270

There is a workaround; specify every fix twice. The result will be the
same, but it will actually work.

I get the feeling I am "holding it wrong". Surely this must have been a
problem for someone in the past. Bruce, how come this has not broken
your surveys?