Add autodiff

Draft for minimal integration of automatic differentiation. Tensorflow and Torch backend were removed (apart from AutoDiffOp.create_tensorlfow())

Only tests without LBM or tf/torch dependencies have been added. This implies that also numeric gradient checking is missing (depends on either tf or torch). Should we really move the backends into separate modules?

Apart from adding auto-differentiation functionality I added two more changes. Would be happy to AutoDiffOp.create_tensorflow() after feedback.

pystencils/autodiff/__init__.py

+from pystencils.autodiff._autodiff_astpair import AutoDiffAstPair
+from pystencils.autodiff.adjoint_field import (ADJOINT_FIELD_LATEX_HIGHLIGHT,
+                                               AdjointField)
+from pystencils.autodiff.autodiff import (AutoDiffOp,
+                                          create_backward_assignments,
+                                          get_jacobian_of_assignments)
+
+__all__ = [
+    'AutoDiffAstPair',
+    'ADJOINT_FIELD_LATEX_HIGHLIGHT',
+    'AdjointField',
+    'AutoDiffOp',
+    'create_backward_assignments',
+    'get_jacobian_of_assignments']