Remove trailing spaces in code (except MbRandom)
Fix small bug in initializing trait values

The conditional clause in the else if statement was identical to the
previous if statement. The consequence was that in some cases, the
node's trait value was not the mean of its two children. Fixed by
swapping variable names.
Choose random internal node starting at 0

There is no reason to start at 1, unless the intent was for the root
to never be chosen. However, because the internalNodeSet is a set and
not a vector, starting at 1 doesn't do anything because nodes in a set
are ordered by their memory location, regardless in the order they're
added to the set.
Make internal nodes a vector rather than a set

A vector is more efficient than a set for most purposes, which includes
how the internal nodes list is being used here.