Moved dcdm parameters around inside background structure.
Added support for omega_dcdm
Corrected bugs related to dcdm
updated version number to 2.3.1
Merge branch 'devel'

* devel:
  updated version number to 2.3.1
  Corrected bugs related to dcdm
  Added support for omega_dcdm
  Moved dcdm parameters around inside background structure.