Allow user to set multiple trees as target

[ArtPoon]

Complication: tree height parameter t_end will have to be fixed for each target tree in the list. i.e., not treated as a parameter to be updated by the proposal.

[ArtPoon]

• In Kamphir:simulate(): For each target tree, need to simulate replicate trees with matching numbers of tips, tip heights, and tree height. Needs to return a list of lists containing Tree objects.
• Kamphir:compute() needs to iterate over the list of lists of simulated trees returned by simulate and calculate kernel scores for each target tree.
• It may be easiest to handle this from the level of evaluate() and pass tree-specific objects so that we hardly need to change simulate or compute.
• The downside to this is that we incur the same overhead cost to setting up a simulation with each target tree -- but I don't think this cost is that bad.