HMC Clock Model doesn't produce any mention or citation

[rambaut]

The HMC relaxed clock model uses only generic XML elements so doesn't get reported as being used in the stdout or the citations.

Possible options:
Create alternative parser for ArbitraryBranchRates which adds a comment to the screen log and a citation to the object.

Create a derived class from ArbitraryBranchRates for this purpose.