Merge branch 'interpolation-refactoring' into 'master'

pystencils/backends/cbackend.py

pystencils/backends/cuda_backend.py

pystencils/fd/derivative.py

pystencils/fd/finitedifferences.py

pystencils/field.py

pystencils/gpucuda/cudajit.py

pystencils/gpucuda/texture_utils.py

pystencils/interpolation_astnodes.py

pystencils/math_optimizations.py

pystencils/simp/assignment_collection.py

pystencils/transformations.py

pystencils_tests/test_interpolation.py

pystencils_tests/test_opencl.py

pystencils_tests/test_sympy_optimizations.py