It is not possible to use save() on TreeMan objects
with node matrices. Node matrices are bigmemory matrices and are therefore outside
the R environment, see bigmemory documentation for more information. Saving and loading
a bigmemory matrix may cause memory issues in R and cause R to crash.
This function can safely read a TreeMan object with and without
a node matrix. saveTreeMan() function stores the tree using the serialization format
and the node matrix as a hidden .csv. Both parts of the tree can be reloaded to an R environment
with loadTreeMan(). The hidden node matrix filename is based on the file argument:
file + _ndmtrx
Reading and writing trees with saveTreeMan() and
loadTreeMan is faster than any of the other read and write functions.