Document time dependent source models

Time dependent sources are not necessarily spherical, so we rename
shells as cells in the documentation.
Replace --shell by --cell in plabun

Shells are now named cells in the documentation, so we replace the
--shell option by --cell in plabun for a sake of clarity. The --shell
option is left for backward compatibility, but the command prints a
warning if it is used.
Replace --shell by --cell in plroute

The shorthand for --cell is -c, but this was also used for
--chmfile. Use -m as a shorthand for the later.
Replace shell by cell in libastrochem.py
Replace shell by cell in output.c
Update the NEWS.md file

This closes #20.