Substitution model goals

[rvosa]

• [x] make sure all models are suitably POD documented
• [x] verify that the serialization for different programs is correct
• [x] optional: think about round-tripping, e.g. via NeXML/RDFa

[rvosa]

There's now POD for the DNA base class, but not for any of the substitution model subclasses.

[rvosa]

The model subclasses now have POD as well.