Improve style consistency

[MaxThevenet]

Similar patterns appear across the code, and are sometimes treated in different ways, which lacks consistency. This issue aims at identifying them and deciding on the way to treat each of them:

1. A function needs to do something only if a condition is met. Do we put the condition outside the function (if (m_multi_plasma.IonizationOn() && m_do_tiling) m_multi_plasma.TileSort(bx, geom[lev]);) or in the function (see Fields::AddRhoIons)?
2. A module can be ON or OFF. How do we test it? m_multi_plasma.IonizationOn() or do_MR or InSameTransverseCommunicator or m_multi_laser.m_use_laser?

This list is in construction. If we agree this is worth improving and decide on a convention, we would put this in the doc and slowly implement it in the code, when we stumble upon it. inconsistencies in naming conventions in the input file should be included too.