Optimize new shellwise photo-electric routine

[mainegra]

Selection of interacting element and atomic shell is done using linear interpolation on a logarithmic energy grid. However the interpolation coefficients are calculated on the fly rather than having them pre-calculated before the simulation start, similar to what it is done with the other arrays:

• This is due to the fact that the energy grid is not equidistant. One will have to resize the PE cross section data base to use equidistant energy intervals. This is non-trivial if one wants to keep the structure around the atomic edges,since one could end up using an unnecessary large number of energy points.