Enable Volumetric and Python source terms on 0D elements (aka Points)

This MR enables

1. to use Volumetric source terms as nodal source terms
2. to use Python source terms as nodal source terms

(2) is a new feature.
(1) makes OGS's separate nodal source term implementation obsolete (but we'll keep it for the time being).

• Feature description was added to the changelog
• Tests covering your feature were added?
• Any new feature or behaviour change was documented?