Generalisation of Relaxation rate setter

All methods that have a high number of free relaxation rates have different logic for how the list of relaxation rates is set. First of all, this causes a lot of confusing and unnecessary code and second it is confusing for the user.

In this MR the logic for setting the relaxation rate is generalised and then used in all methods.

• If only one relaxation rate is given by the user it is set as the shear relaxation rate.
• If a relaxation rate for all moments is defined they are just used.
• Otherwise, the conserved moments are relaxed with zero, the next two relaxation rates in the list are used for bulk and shear viscosity and the remaining ones are used to relax a complete group of the same order. If too many relaxation rates are given the list is just cut and the remaining ones have no influence.