BEAMS3D: improvement for handling of ZEFF and NI profiles

[kudav]

Currently, when specific ion density profiles are given, a ZEFF profile is calculated from them and the atomic information also given.
The reverse does not happen however, but we could have the code calculate it either by assuming an impurity (e.g. Carbon and making this transparent in the output) or by using NI_AUX_M and NI_AUX_Z to calculate the ion profiles from zeff. This becomes important when using the ion densities for calculating collisions.

[lazersos]

This is done on purpose. If you need a value for the code to run then you should specify it explicitly. I don't like the idea that such things are hidden from the user. Carbon is a bad choice for a Tungsten machine and vice versa.