The LTE setting of masering lines now happens in a while loop

[ThomasCeulemans]

It appears that previous attempts at filtering out population inversion forgot to set some levels to LTE, due to only doing a single pass over all populations. To make sure this is fixed, the following heavy-handed approach has been used:

• do two passes (respectively starting from the lowest and the highest level) and afterwards set all affected levels simulatenously to LTE
• repeat this the until no extra levels need to be changed

[ThomasCeulemans]

Note to self: also check if Ng-acceleration could cause some additional issues.
Edit: Ng-acceleration seems to have no bad influence on the convergence