Allow for multiple independent experiments to run in parallel.

Allow for multiple independent experiments to run in parallel.

Each experiment is dynamically assigned an experiment id (eid) which
it then uses throughout its lifetime.  By default the eid is
constructed by appending the process id of the tcl interpreter to
the prefix "imunes_".  All vimages and netgraph nodes corresponding
to virtual nodes in an experiment are created as child nodes of
a "controlling" vimage which has the same name as eid.  All virtual
node specific temporary directories / files are created as
subdirectories in /tmp/$eid directory.

vimageCleanup function is modified to destroy / cleanup only those
vimages / netgraph nodes bellow and including the controlling
vimage for the given experiment.  Temporary files in /tmp/$eid are
no longer automatically deleted.

Bug found by:
Submitted by:
Reviewed by:
Approved by:
Obtained from: