Add radial DG discretization

[jbreue16]

This PR adds an arbitrary order DG discretization for the radial flow models

Todo

• Implementation of radial DG with its own convection dispersion operator, similar to FV
• Incorporation of this operator in the rLRM and ColumnModel1D unit
• Add tests
  • Python code for EOC tests, implemented in CADET-Verification
  • C++ tests in CADET-Core. This will only be done after the python EOC tests and @jbreue16 will help you.
• Add documentation. To be done in the end, @jbreue16 will help you

[jbreue16]

Hi Yuvj, now that you have already implemented rLRMP and rGRM as separate units, I think you can first (EOC) test this in pyhton before refactoring into the ColumnModel1D.
Also, there are a couple of unrelated changes.