Method to write a .geom file based on an existing one

[takluyver]

Prompted by discussion with @turkot on PR #114:

EXtra-geom has methods to read and write .geom files, but any information that we want to write back out must be parsed, kept as metadata, and serialised again. This is tricky, because the CrystFEL geometry format is designed to be human writable: comments are used generously, panels can have arbitrary names, there are different ways to specify the same values, etc. All this flexibility is good for human editors, but more work to handle automatically.

Tom White's scripts (e.g. transfer-geom) get round this by using an existing geom file as a basis, and only replacing specific parts. We could do something similar with data from an EXtra-geom object.

What should it modify? My initial thought is to replace only the fields relating to positions (corner_x, corner_y, coffset, ss and fs). I suspect that trying to do anything else gets complicated quickly.