This commit adds a new helper class, RstNetwork, that loads the raw
data representing the extended network model from restart vectors
IBRAN, INODE, RNODE, and ZNODE. We support loading the information
that is currently output to those restart vectors, but those do not
include every option supported by the input keywords BRANPROP and
NODEPROP.
The RstNetwork type supports querying whether or not the extended
network model is active and, if so, to get iterable collections of
branches and nodes that are geared towards constructing ExtNetwork
objects.