Improve mass insertion logging

[kyleaoman]

Currently the total mass in the source is recorded in the log, and then the fraction of that mass that is inserted in the cube. This often leads to low fractions simply because much of the mass is outside of the datacube aperture. Should additionally record the mass in the source after it is pruned to the desired aperture, and also log fractions of this mass. It's useful to flag for the user both (i) when much of their source is outside of the aperture (which they might not have expected), and (ii) when significant amounts of mass that are in aperture don't end up in the cube (probably indicating that something has gone wrong in the approximations used in the SPH kernels or similar).