Improve compression in OMF

Adds several features that can all be enabled separately:
- DEFAULT_PEPLIB: OMF stores a ResidueDefinition object for each observed
                  unique compound. No need to dump residue definitions for
                  standard amino acids.
- LOSSY: Reduce coordinate accuracy to 0.1
- AVG_BFACTORS: Optimization specific for models, where all atoms of a residue
                have the same bfactor => store one bfactor per residue instead
                of one bfactor per atom
- ROUND_BFACTORS: Round bfactors, i.e. 42.42 => 42.0
- SKIP_SS: Don't dump secondary structure, loaded objects assign COIL to
           each residue
- INFER_PEP_BONDS: Intra-residue bonds are stored in ResidueDefinition objects.
                   Inter-residue bonds need to be stored separately. No need
                   to do that for peptide bonds as this can be inferred on the
                   fly when loading