Extend 'plotPrior'

[Caetanods]

• The 'plotPrior' function now assumes that the priors for the two rate regimes are the same. Need to extend this to work with different priors for each regime.
• The function only plots the prior distribution for the evolutionary rate matrix. Need to add an option to be able to also plot the prior for the root value. One way would be to have a option choosing between the plot of the prior for the root and the prior for the rate matrix.
• Also would be great to plot the prior in a bounded region of the parameter space, such that I could plot the posterior alongside the prior. Right now you need to do separated plots.

[Caetanods]

Plot the prior for the root was implemented on commit 7aa844b

[Caetanods]

This tread is duplicated with the enhancements wish list.