Ehn: utils.material functions now mutlidimensionnal

Compositional map of any dimension can be now changed to atomic to weight percent. The density map can be calculated.

Shorcut added for EDSSpectrum, that will be used with the result of quantification Cliff-Lorimer  #389
Lines 147 to 159 are the same in `weight_to_atomic`, `atomic_to_weight` and `density_of_mixture_of_pure_elements`, wouldn't it make sense to create a new (private) function to perform the task and call it from these three functions?