Prepare release of v1.4.9

See merge request !40

Fix failing tests

See merge request !39

A number of tests failed, because there's check in the LSMTool source code
on the version number of SciPy. This check breaks if the version number is
that of a release candidate. Now, I'm not sure why `python setup.py test`
pulls in a pre-release version of SciPy, but it easier to fix this in the
source code.

TODO: the use of `distutils.version` is deprecated; it shoud be replaced
      with `packaging.version`

RAP-4 Update links

Closes RAP-4

See merge request !38

Updates the documentation to Read the Docs.
Updates the source to ASTRON's GitLab.

RAP-4 Deploy documentation

Closes RAP-4

See merge request !37

The documentation is generated by Read the Docs
https://readthedocs.org/projects/lsmtool/

Close all plots when done

Add script for fixing Gaussian position angle in existing skymodels

See merge request !34

Prepare for release of v1.4.8

See merge request !36

Fix bug in concat when order of spectral index differs

See merge request !35

RAP-457 Implement basic CI

See merge request !33

Add option to query LoTSS catalog

Fix reading of patchless sky models with Astropy < 4.1 (fixes #29)

Fix table column converters for astropy versions >= 4.1 

Fix logic of output check

Fix check of blank values