Made existing documentation doxygen-readable

For the computational electrophysiology module.
Also removed some unused functions.
Simplify checkpoint read/write for swapstate.

We do not need to be compatible with older (4.6.x) computational
electrophysiology versions.
Merge branch 'comp_el_master' of ssh://gerrit.gromacs.org:29418/gromacs into comp_el_master