Implement a method for using the least amount of CLV vectors

[xflouris]

Implement a pll_update_operations_minimal that takes a list of operations and an integer N and re-assigns parent_clv_index, child1_clv_index and child2_clv_index of all nodes in order to use the minimal amount C of CLVs, which is, in the worst case, log(n)+2.

Another useful function would be: Given a tree, and not requiring a partition instance, compute the minimal number C, such that this amount may be passed later on to the pll_partition_create function, in case we are working with very large datasets.