Better documentation for tergm control settings

[martinamorris]

The documentation on the control settings leaves much to be desired. This could be improved with a vignette or separate standalone help page in the package.

Here is the tortuous current path:

The set.control.tergm function can't be queried directly:

?EpiModel::set.control.tergm and the ?? version both come up empty

?EpiModel::netest points to tergm::simulate.network and

tergm::simulate.network points to tergm::simulate.tergm