openmp changes to allow projection matrix parallelisation

At present, for SPECTUB there is a problem that when doing forward projection, gradient calculations etc, the code wants to use multi-threading, but the SPECTUB matrix cannot (unless all views are cached, but it is then still very ineffective).

On the other hand, the matrix calculation itself could be parallelisable.

One way to achieve this would be to

• create an extra CMake variable, STIR_OPENMP_PROJECTIONS (set by default to STIR_OPENMP), change the code in distributable.cxx, ForwardProjectorByBin.cxx etc to use that variable, such that it can be disabled, while still using OPENMP in the rest of the code.

• create another variable STIR_OPENMP_MATRIX (set by default to false) that enables then any projector parallelisations.

Another advantage of this would be that it would enable MPI/GPU and OPENMP to be used together.

CMake variables would need to be set in src/CMakeLists.txt and cmake/STIRConfig.h.in

Start with src/CMakeLists.txt and cmake/STIRConfig.h.in to trace how STIR_OPENMP is configured, then inspect distributable.cxx and ForwardProjectorByBin.cxx. Done means projection OpenMP can be disabled independently, matrix parallelisation can be enabled separately, and the requested defaults support combining MPI, GPU and OpenMP.