recursionDown uses the function updateDown to compute
daughters rows of matrix param.
Usage
recursionDown(phy, params, updateDown, ...)
Arguments
phy
Input tree, in cladewise order.
params
Matrix of parameters to update by the recursion
updateDown
Function to be used for the update
...
Arguments to be used by the function updateDown
Value
Matrix of parameters updated.
Details
This functin is to be used in other more complex function that need to
update a quantity from the root to the tips of a tree. Note that the
input tree must be in claddewise order.