add handle parameter to CreateEntityFromView

If the handle is valid, the atoms, residues, chains, bonds an torsions
are added to the handle instead of creating a new one. Useful to merge two entity handles into one.

git-svn-id: https://dng.biozentrum.unibas.ch/svn/openstructure/trunk@1765 5a81b35b-ba03-0410-adc8-b2c5c5119f08