Factory Loader Fix

An issue has been discovered in template and parser node loading. If a node fails to pre-instantiate using standard Python class loading, then the loading of all remaining nodes fails and the parser is left with only the nodes loaded to that point.

This manifests itself in strange errors, with grammars no longer working that previously worked.

The issue was discovered when python-dateutil was added to requirements.txt several 10s of releases ago, but I did not make it clear this had been added or that it was a python dependency.

All users should either carry out

pip3 install -r requirements.txt
or
pip3 install python-dateutil