PinholeSPECTUB code uses global variables

[mastergari]

The underlying *PinholeSPECTUB* code uses the following global variables:

https://github.com/UCL/STIR/blob/43b674789ca0e8865b33aa2fcded5992c8f2ca4f/src/recon_buildblock/ProjMatrixByBinPinholeSPECTUB.cxx#LL86C1-L93C2

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 change the global variables to class members as done with the SPECTUB code (see issue #1108 and fix #1169).