Add beam offset to yml2c instrument files

The main aim of this MR is to add a direct beam offset to the instrument state file, since this value will generally not change. The offset values can be accessed via the Monochromator class, and is not automatically applied so as to preserve test integrity. The offset can be applied via the method InstrumentStateSet::applybeamOffset.

In the GUI, the default offset values are used to set the spin boxes that control direct beam adjustment to a sensible value.

I have also refactored the boilerplate direct beam adjustment code in the three widgets it appears into a single unified control set that can be added to the left "control" widget on any subframe.

Resolves #604 (closed), #605 (closed)