remove dependency on networkDynamic, ergm, tergm, and EpiModel

[chad-klumb]

Companion PR to statnet/networkDynamic#16.

Updates networkLite package to reduce dependencies.

[chad-klumb]

Steps to complete the separation of networkLite from EpiModel:

1. Review and merge this PR. Push the result to CRAN as a new networkLite package.
2. Review and merge import networkLite and define as.networkDynamic.networkLite statnet/networkDynamic#16; push the result to CRAN.
3. Review and merge split out networkLite as a separate package EpiModel#770; push the result to CRAN.
4. Review and merge EpiModel/EpiModelHIV-p#176. (Not on CRAN.)

Completing step 1 should not introduce an immediate need to update other packages on CRAN (as nothing will depend on networkLite, initially). Out of an abundance of caution I would recommend coordinating steps 2 and 3 so that they can be carried out in quick succession, just in case. Step 4 is very small and simply keeps EpiModelHIV-p@main working with main/CRAN versions of other packages.

These changes are mostly independent of the multinets changes but I would recommend choosing one or the other to complete first rather than interleaving the two sequences of merges/pushes.