[Therion] 2 fixes for a point with standard deviation

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?