Automatic Finite Volume and Volume-of-Fluid discretization

I don't yet have a good test for volume-of-fluid and the source term, but @alexander.reinauer requested the code because he needs it now. I'll add the missing tests at some later point.

pystencils/fd/__init__.py

@@ -3,10 +3,11 @@ from .derivative import (
     expand_diff_full, expand_diff_linear, expand_diff_products, functional_derivative,
     normalize_diff_order, zero_diffs)
 from .finitedifferences import Discretization2ndOrder, advection, diffusion, transient
+from .finitevolumes import FVM1stOrder, VOF
 from .spatial import discretize_spatial, discretize_spatial_staggered
 
 __all__ = ['Diff', 'diff', 'DiffOperator', 'diff_terms', 'collect_diffs',
            'zero_diffs', 'evaluate_diffs', 'normalize_diff_order', 'expand_diff_full', 'expand_diff_linear',
            'expand_diff_products', 'combine_diff_products', 'functional_derivative',
            'advection', 'diffusion', 'transient', 'Discretization2ndOrder', 'discretize_spatial',
-           'discretize_spatial_staggered']
+           'discretize_spatial_staggered', 'FVM1stOrder', 'VOF']