Implement template member function in proposalvariance class

[mmulvahill]

Currently this uses an extra layer of abstract classes

class MarginalProposalVariance : public ProposalVariance { ... };

but since the only diff between Marginal and Joint is the type of the pv object (arma::mat vs double), it might be more straightforward to implement the get.pv(), adjust.pv as a template member functions. Only unsolved issue would be setting the type (and size for arma::mat) of pv.