Tip sampling support in BEAUti #20
Description
Currently, the procedure is to specify an MRCA prior and select tipsonly=true, save the XML, and replace TipDatesRandomWalker with SampledNodeDateRandomWalker in a text editor.
Having an SampledAncestorMRCAPrior that derives from MRCAPrior and an input editor associated with it, allows this process to be all done in BEAUti, without having to deal with a text editor.