include delta-solutions

[raphaelquast]

A possibility for choosing delta-distributions as phase-functions and/or BRDF's would
be nice. This could be achieved by directly including the exact solutions which can easily be gained for delta-like scattering-distributions.

[raphaelquast]

While the solutions for the interaction-term for delta-like brdf's or phase-functions can be gained easily,
incorporating it in the model (in a general way) would require some effort in order to allow using the functions within linear-combinations since one has to come up with a way to distinguish between delta- and non-delta-like functions when calculating the interaction term which possibly needs some extensive changes within the code....

Thus, I would again take one step back and leave this as a future possibility...