[IMP-dev] density map volumetrics insertion

Added densityMapVolumetrics{.cpp/.h} files to the IMP::em module They provide - functions to compute mass/volume of a densityMap at a given threshold - functions to compute the threshold one needs to apply in an EM map to attain a given mass/volume In addition, one can use different protein density values in these computations. The default density being used remains 1/1.21 Da/A^3, the value that was used since then.

A small duplication issue remains to leverage : DensityMap::approximate_molecular_mass() should probably disappear or be remapped on compute_molecular_mass_at_threshold( DensityMap* m, Float threshold);

Shall I do it ?

--Ben(S)