Use the same angular momentum vector for all surface density plots

[sylviaploeckinger]

[I'll drop this here so that I do not forget.] Currently the new surface density plots for the HI-H2 transition do not use the same angular momentum vector for the projections as the other plots (e.g the KS plots) (see discussion in #12). They are calculated in a very similar way, but the mask is slightly different and for consistency it would be good if they would use the same projections.