v3.3.0

• Added a new acquisition model SPECTUBMatrix for (simple) usage in SPECT.
  sirf.STIR.ImageData has now a way to set/get the modality.

• interactive Python demos superseded by SIRF-Exercises notebooks removed.

• arguments setting number of CG iterations and verbosity of output
  of acquisition models method norm() added.

• path for ISMRMRD shared library when generating MR data fixed.

• extension processing in ISMRMRDImageData::write fixed.

• Python int array check/conversion ensuring C++ compatibility added.

• C/Python interfaces for computing prior value added.

• added CITATION.cff file (replacing .zenodo.json`)