Improve IMP metadata handling and output
Addresses #141 (closed) and some further tweaks:
- Metadata lines in .model files do not need to end with a semicolon anymore
- Simplified parsers for metadata in limpet_fe.py and limpetcommon.py
- Ionic model metadata fields were cut if they contained a colon
- Made ionic model metadata fields consistent (sometimes singular, sometimes plural was used; Author/Authors)