AST-1416 Base OSKARElementResponse on beam functions in ska-sdp-func

Use functions from the processing function library (ska-sdp-func) to evaluate the oskar beam. The dependency on the OSKAR repository is removed en replaced by a dependency on ska-sdp-func This dependency is provided in CMake by FetchContent, instead of a submodule as was OSKAR

There was already a test for OSKARElementResponseSphericalWave which still succeeds after switching to the implementation in the processing function library.

OSKARElementResponseDipole has been converted too, but this class was and still is untested. This class appears not to be in active use. For example the dipole length is hard coded to 1m and marked with a TODO.