Top level Python module is now "openmm" (#3000)

* Top level Python module is now "openmm"

* Updated module names in examples

* Updated module names in documentation

* Updated module in CI scripts

* Added deprecation warning

wrappers/python/tests/TestUnits.py

 """
-Tests the functionality in the simtk.unit package.
+Tests the functionality in the openmm.unit package.
 """
 from __future__ import division
 
-from simtk import unit as u
+from openmm import unit as u
 import copy
 import math
 import unittest
@@ -672,7 +672,7 @@ class TestNumpyUnits(QuantityTestCase):
 
     def testNumpyIsString(self):
         """ Tests the internal _is_string method with numpy Quantities """
-        from simtk.unit.quantity import _is_string
+        from openmm.unit.quantity import _is_string
         a = np.array([[1, 2, 3], [4, 5, 6]])
         self.assertIsInstance("", str)
         self.assertTrue(_is_string(""))

wrappers/python/tests/TestVec3.py

 """ Tests the Vec3 object """
 from unittest import TestCase
-from simtk.openmm import Vec3
+from openmm import Vec3
 
 class TestVectors(TestCase):
     """ Tests the Vec3 type """

wrappers/python/tests/validateConstraints.py

-from simtk.openmm.app import *
-from simtk.openmm import *
-from simtk.unit import *
+from openmm.app import *
+from openmm import *
+from openmm.unit import *
 
 def validateConstraints(self, topology, system, constraints_value, rigidWater_value):
     """ Given a Topology, System, a value for 'constraints' and a value for