Grammages are not put in the correct units

In multiple places, grammages (like the grammage value of a slice) are put in g/cm2 but not converted using the units package. This makes sense, because all operations currently are done in g/cm2. Also, the unit value of g/cm2 is really weird. But this might be confusing for users. So either we document this properly, which still leaves room for error, or we change our unit system such that g/cm2 is 1 and hence it is consistent.