Fixing missing dependencies to install wsclean

This merge request adds the following:

• gsl dependency for wsclean
• boost variants necessary to build wsclean
• add wsclean executable to PATH
• automatically source OPENBLAS_NUM_THREADS=1 in the wsclean run environment
• added idg 0.8.1 to be visible using spack info idg (for people that want to rely on releases/tags)
• preferred idg/wsclean versions are explicitly indicated (this is optional but improve readability)

Additional comments:

• I would simplify the README.md with a very simple example, similar to:

spack:
  specs:
  - wsclean@latest %gcc@9.3.0 
  view: false
  concretization: together
  repos:
  - ../external/schaap-spack
  - ../external/hdf-spack

  modules:
    enable:
    - tcl
    tcl:
      hash_length: 0
      naming_scheme: ${PACKAGE}/${VERSION}
      all:
        environment:
          set:
            '{name}_ROOT': '{prefix}'

• Then, I would create a Wiki page to describe how to use Spack to have a reproducible environment, e.g. das-6-server.