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
For "composition" of vacuum. If in a map a pixel composition is (0,0) in weight percent being in recorded in vacuum, it would be nice to have (0,0) as atomic percent.