Implementation of nodal source terms

- Theory for analytical solution.
- Comparison-images to analytical solution.

The reference files are created with the analytical solution. For the
special setup with a dirac source term at position xi, eta the following
analytical solution in 2 dimensions is valid:

u(x,y) = ln(sqrt((x-xi)^2+(y-eta)^2))/(2 * Pi)

This avoids a linker error.

RichardsComponentTransport again

Use variables for indices and size for accessing local matrices.

Summarize several times used mathematical expression
into a variable.

Change the assembly for the pressure related entries of the local right hand side.

Change the assembly of the local mass matrix for the pressure Mpp.

Change the assembly of the local matrix for the pressure Kpp.