
Tests the plugin feature of Tkenv:
- that .tcl files get loaded from .plugins (should test TKENV_PLUGIN_PATH 
  env. var too!)
- registerPlugin <pluginname> function
- <pluginname>:newNetwork callback
- that plugins are able to access some Tkenv functionality, such as 
  accessing/changing parameters, running/stopping the simulation, 
  hiding the main window, etc. 
  
TODO: create a well-defined API plugins to use (otherwise Tkenv changes will
affect plugins)