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
This error won't help the user know what the problem is. What about:
"The signal %s contains more than one element but this function requires only one element per signal." % weight.metadata.General.title