Remove interpolator

...books/05_tutorial_phasefield_spinodal_decomposition.ipynb

doc/notebooks/demo_derivatives.ipynb

pystencils/astnodes.py

pystencils/backends/cuda_backend.py

pystencils/cpu/kernelcreation.py

pystencils/fd/finitedifferences.py

pystencils/gpucuda/cudajit.py

pystencils/gpucuda/kernelcreation.py

pystencils/interpolation_astnodes.py

pystencils/kernelcreation.py

pystencils/transformations.py

pystencils_tests/test_field_coordinates.py

pystencils_tests/test_interpolation.py