Add piece-wise constant radial dependence of parameters to 2D DG discretization

[jbreue16]

This PR adds piece-wise constant radial dependence of parameters to the 2D DG discretization.

TODO

• change parameter sizes from radNPoints to radNElem
• radially dependent porosity
• radially dependent velocity
• radially dependent particle type volume fraction
• radially and component dependent axial dispersion
• radially and component dependent radial dispersion
• user-defined element-spacing to improve control over radial inhomogeneity and refinement
• column length parameter sensitivity (radius wont be fixed?) -> see aux equations todo in residualImpl
• Move axial and radial dispersion to main equation and allow sensitivities
• Add Jacobian test with radially variable parameters
• arbitrary polynomial degree per radial element?
• Jacobian test with radially variable partypeVolFrac

Remenants from #160 :

• consistent initialization with AD Jacobian and req binding fails
• FD Jacobian pattern comparison fails
• Add documentation (unify the modelling documentation of GRM2D and LRMP2D)
• radial coordinate was > column_radius by some 1e-18 .. which was also written to the output, triggering an exception in the evaluation.. should there be something different on the c+ side?