Returns the Robinson-Foulds distance between two trees.

## Arguments

- tree_1
`TreeMan`

object- tree_2
`TreeMan`

object- nrmlsd
Boolean, should returned value be between 0 and 1? Default, FALSE.

## Details

RF distance is calculated as the sum of partitions in one tree that are not shared by the other. The maximum number of split differences is the total number of nodes in both trees (excluding the roots). Trees are assumed to be bifurcating, this is not tested. The metric is calculated as if trees are unrooted. Parallelizable.