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/simtk/openmm/app/data/charmm_polar_2013.xml → wrappers/python/openmm/app/data/charmm_polar_2013.xml

@@ -9208,7 +9208,7 @@
   <Particle type1="809" type2="556" type3="554" type4="559" type5="560" charge="-1.45087" polarizability="0.000699" thole="2.399" aniso12="0.7229" aniso34="1.265"/>
  </DrudeForce>
  <Script>
-import simtk.openmm as mm
+import openmm as mm
 harmonicBondForce = [f for f in [sys.getForce(i) for i in range(sys.getNumForces())] if type(f) == mm.HarmonicBondForce][0]
 nonbondedForce = [f for f in [sys.getForce(i) for i in range(sys.getNumForces())] if type(f) == mm.NonbondedForce][0]
 numAtomClasses = 128