Change the mutationRate input in SiteModel to a Function

[alexeid]

This will allow the possibility of applying a ScaledDirichlet prior to a multi-dimensional relative rates parameter and then slicing out the components to the site models of each partition.

https://github.com/CompEvol/beast2/blob/b7a1c6d9920ca2847207aa3c1c155a74393456d9/src/beast/evolution/sitemodel/SiteModel.java#L54