Add loading efficiency to xsection plot

[MattBrst]

I would like to add the loading efficiency to the cross section plots.

Example from the benchmark with the following line of code:

ml_leff[0].plots.xsection(params=True, names=False, labels=False, sep="\n")

So basically non-zero loading efficiencies for aquifers are plotted.

[dbrakenhoff]

Good addition, but could you modify it to use the symbol $\beta$, similar to Mark's paper? Looks a bit cleaner than load eff written out, though it is a bit more obscure as to what that means.

We could consider a small list of symbols somewhere to help users understand what's what. Or at least have the loading_efficiency notebook mention $\beta$.

[mbakker7]

And to be one step ahead in the process, the loading efficiency of the leaky layer is on its way. So can we include that right away as well?