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);

    --Ben(S)