Forsight/backsite export format improvement

[AndrewAkinson]

Although I've only done this for the Therion export, I assume it is the same in other exports.

If you take a forward and backward leg on the same leg it appears as a single leg (which is how sexytopo handles legs so correct)

However, on export it is also a single leg (set of numbers) For three shots in the same direction the original shots are in {} The export should do this at the very least. However, for legs where a fore and backsight where taken this is an important improvement on blunder removal and maybe reading accuracy that survex does (I believe) take into account. Survex is designed to input fore/backsight readings. From the survex manual the DATA command may have the following BACKTAPE BACKCOMPASS BACKCLINO I'm not sure what survex does if given a leg forward and then backwards, but from survex 1.2.17 it will average readings that are adjacent to each other.