Molsystem

* Added the ability to create a system from an OpenEye molecule and vice versa.
This gives access to the OpenEye toolkit for generating conformers, etc.
* Added the ability to create a system from a SMILES string and create the SMILES
string from a system using OpenEye.

* Any properties on the system and configuration are optionally added to the
Openbabel Molecule object for e.g. writing to an SDF file. This adds the units of
the properties explicitly

* Fixed bug where the coordinates from PubChem were accidentally the 2-D rather than
3-D coordinates.

* Existing instances of systems did not correctly update when the default
configuration was changed. This is release fixes the problem.

* Fixed bug where creating molecules consisting of just hydrogen failed because
RDKit by default ignores all hydrogens when reorienting the molecule.

* RDKit seems more robust, and also the atom typing uses RDkit, so compatibility is
important.