Fix for the structure.com user function.
This fixes bug #22860 (https://gna.org/bugs/?22860), the failure of the structure.com user function after calling structure.delete. The number of models in cdp.structure is now counted and if set to zero, RelaxNoPdbError will be raised.
Authored by: bugman 2014-10-23
Parent: [r26373]
Child: [r26375]