Integrate GPU gridding processing function into processing function library

The GPU version of the 3D nifty gridder with python wrapper was tested and compared with other gridders. Some inconsistencies were discovered ( https://confluence.skatelescope.org/display/SE/HIP-13%3A+CPU+and+GPU+gridders+comparison ) and corrected by the NZAPP team. A python wrapper for the GPU version of the 3D nifty de-gridder was written, tested and documented. The GPU version of the 3D nifty de-gridder was also tested ( https://jira.skatelescope.org/browse/HIP-43 ). The tests revealed some remaining structures in the results, which were is not present in the standalone version of the 3D nifty de-gridder as reported by NZAPP. The double-precision version suffers from lower precise results, which might be caused by the observed structures. Both functions were integrated into the RASCIL imaging pipeline ( SP-1697 ) as an example of the possible use. Repository for the gridder and de-gridder python wrappers: https://gitlab.com/ska-telescope/sdp/ska-gridder-nifty-cuda/-/tree/sim-874-python-wrapper Changes to the RASCIL pipeline: https://gitlab.com/ska-telescope/external/rascil/-/tree/hip-17_wagg_context This work was presented during 11.6 demo session: https://confluence.skatelescope.org/display/SE/2021-08-25+DP+ART+System+Demo+11.6 Future work: Investigation of remaining structures in single and double precision and lower than expected precision of double-precision version of the de-gridder.