Calculates the chemical potential of species consisting of more than one residue, like salts.

I'll just open this MR if anyone is interested in discussing this.

When multiple selection groups are entered, the mean value of the number density as well as the total mass of the selection groups is used to calculate the chemical potential.

Example:

maicos density_planar -mu -sel 'resname NA' 'resname CL' 

Also fixes wrong calculation of errors (kT was missing).