Implement textures and software interpolation

pystencils/__init__.py

pystencils/astnodes.py

pystencils/backends/cuda_backend.py

pystencils/backends/cuda_known_functions.txt

pystencils/cpu/kernelcreation.py

pystencils/data_types.py

pystencils/field.py

pystencils/gpucuda/cudajit.py

pystencils/gpucuda/kernelcreation.py

pystencils/gpucuda/texture_utils.py

pystencils/include/pycuda-helper-modified.hpp

pystencils/interpolation_astnodes.py

pystencils/kernelcreation.py

pystencils/spatial_coordinates.py

pystencils/test_gettypeofexpression.py

pystencils/transformations.py

pystencils_tests/test_cuda_known_functions.py

pystencils_tests/test_cudasympyprinter.py

pystencils_tests/test_data/lenna.png

pystencils_tests/test_data/test_vessel2d_mask.png