Reinstate revision 3133 and parts of revision 3419:

  * tasks/abinitio: Split up to ...
  * tasks/molecular-abinitio: ... this and ...
  * tasks/periodic-abinitio: ... this.
  * tasks/molmech: Renamed to ...
  * tasks/molecular-dynamics: ... this.
  * tasks/molecular-dynamics: Removed ghemical and avogadro, added cp2k and
    nwchem.
  * tasks/modelling: Moved nmoldyn to ...
  * tasks/molecular-dynamics: ... this.
