Fixed bug in computation of Fe limitation depth

Fixed a bug in subroutine mobii in npzd_src.F that led to the depth index
of iron limitation (kmfe) being kept at its default value rather than being
computed internally when O_TMM was defined.

Added a print statement in mobi_ini.F to write out kmfe.

Changes to be committed:
  modified:   mobi_ini.F
  modified:   npzd_src.F