Add casacore and python-casacore to the build

Marcel Looserequested to merge
RAP-909_fix-ci-pipeline into master

The latest version of EveryBeam requires Casacore >= 3.6, which is not available in any distribution (yet). So we need to build casacore and python-casacore ourselves now when building the Docker image.

This MR fixes the build, contains a few other improvements and some code clean up.

Closes RAP-909

Edited by Marcel Loose

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