Modify envelope tracker

[austin-hoover]

• Envelope now stores SyncParticle rather than bunch. Added getCharge() method to SyncParticle.
• Moved transfer matrix definitions (quad, drift, etc.) to orbit.utils.matrix.py.
• Added method getMatrix(sync_part: SyncParticle, part_index: int) -> np.ndarray to each AccNode subclass.
  • By default this method will raise NotImplementError.
  • If the element does not affect the coordinates, it should return None.
  • Otherwise it will return the exact first-order transfer matrix.
• Got rid of orbit.envelope.EnvelopeTracker and added envelope tracking routines to orbit.lattice.AccLattice, such as AccLattice.trackEnvelope(envelope: Envelope, index_start: int = 0, index_stop: int = None, history: bool = False). There are additional methods for faster tracking in rings with static elements (by storing transfer matrices rather than recomputing on each turn.
• Added method Envelope.to_bunch(size: int, dist: str) -> Bunch to generate bunch from envelope. Also added bunch argument to Envelope constructor to construct envelope directly from bunch.
• Updated tests (/tests/py/orbit/test_env.py) and examples (/examples/Envelope). I think everything is working as before.