SPECTUB code uses global variables

[KrisThielemans]

The underlying SPECTUB_* code uses the following global variables

STIR/src/recon_buildblock/ProjMatrixByBinSPECTUB.cxx

Lines 63 to 67 in 877511b

	namespace SPECTUB {
	wm_da_type wm;
	wmh_type wmh;
	float * Rrad;
	}

This creates

• problems when creating 2 different matrices
• trying to use openmp pragmas, as it probably doesn't know what to share.

The suggested solution is to

• make the 3 global variables member variables of ProjMatrixByBinSPECTUB
• change the signature of relevant functions in SPECTUB_* to include (ideally const) references to the variables

This is slightly painful, but clearer than the alternative of making the SPECTUB_* functions methods of ProjMatrixByBinSPECTUB.

This would then also have to be done for #1100 (but after merging that).