fix(simulation packages): *** Breaks interface *** (#1394)

* fix(simulation packages): The gnc and mvr simulation packages are now generic packages

* fix(simulation packages): Added tests for simulation packages and subpackages

* feat(new tutorial): Added tutorial showing how to work with multiple models in a simulation.  Includes example of how to connect sub-packages to a exchange package.

* fix(code formatting)

* fix(formatting)

* test(): testing changes with updated mf6 tests

* testing

* testing

* testing

* rollback test code

autotest/t504_test.py

@@ -590,6 +590,8 @@ def test045_lake1ss_table():
 
     # change some settings
     model = sim.get_model(model_name)
+    lak = model.get_package("lak")
+    laktbl = lak.get_package("laktab")
     laktbl = model.get_package("laktab").table
     laktbl_data = laktbl.get_data()
     laktbl_data[-1][0] = 700.0