Add documentation and refactor distributed_init

This MR:

• Documents distributed_t
• Refactors distributed_init
• Adds several issues to states_elec_t, indicating refactoring tasks

Prior to removing:

    integer                     :: lnst                 !< Number of states on local process.
    integer                     :: st_start, st_end     !< Range of states processed by local process.
    integer, allocatable        :: node(:)              !< To which node belongs each state.

from states_elec_t, I've tracked down their definitions and consequently documented distributed_t

The data st_start, st_end, node and lnst are all contained in dist, and so can be deleted with no loss of information. This will be done in a subsequent MR.

This closes issue #825 (closed)