This function appends the transmission matrix created during
infection.net and infection.2g.net.
set_transmat(dat, del, at)Master list object containing a networkDynamic object or
edgelist (if tergmLite is used) and other initialization information
passed from netsim.
Discordant edgelist created within infection.net and
infection.2g.net.
Current time step.
This internal function works within the parent infection.net functions
to save the transmission matrix created at time step at to the master list object
dat.