Settings for detector type parameters

[cmarooney-stfc]

In tobyfit-dG*_resconv_init functions, the detector population is now obtained from experiment_info detector_arrays rather than being built on the fly anew from detpar. There still needs to be more information to determine what the type of detector it (He3Tube, TobyfitClassic), and for some types additional parameters (pressure and wall thickness for He3Tube.)

With issue #1298 and PR #1331 this information is made available via a static method tobyfit_defaults of IX_det_abstractType. Here persistent values are used to set these parameters once e.g. in a test suite constructor or command line session, and reused to complete the detector construction for all detectors found. Typically these have been stored in sqw files retireved for the tests.

This ticket covers the need to replace this interim solution by actual values from nxspe files. It may still need to be retained if the nxspe files remain unpopulated for some runs.

As an additional point, He3Tube pressures and wall thicknesses are currently hardwired at 10atm and 6.35e-4m respectively. This has been used in a variety of tests but their origin is undocumented. Information on the source of this data should be added.

The code for DGdisk and DGfermi could also usefully be refactored to remove common code to a common location