Corrected passing of mmax parameter in multipole_electrostatics

[robin-dahl]

I am currently working on using ddX together with a multipole-expanded solute density. While implementing this, I noticed that in multipole_electrostatics the argument mmax is strictly passed as mmax=0 to its subsequent subroutines. This prevents setting up a suitable potential beyond monopoles.

I corrected the corresponding arguments to use the more general mmax, which should already be defined when the host code calls the function. I suppose this was the original intention, since other routines in the ddx_multipolar_solutes module pass mmax as well. The routines multipole_electrostatics_1–multipole_electrostatics_3 likewise expect mmax as an argument and do not need to be restricted to 0.

This should therefore be a straightforward bug fix and is not expected to introduce further issues.