Implemented node collection

pystencils/__init__.py

pystencils/backends/cbackend.py

pystencils/config.py

pystencils/cpu/cpujit.py

pystencils/cpu/kernelcreation.py

pystencils/functions.py

pystencils/kernel_contrains_check.py

pystencils/kernel_decorator.py

pystencils/kernelcreation.py

pystencils/node_collection.py

pystencils/transformations.py

pystencils/typing/cast_functions.py

pystencils/typing/leaf_typing.py

pystencils/typing/utilities.py

pystencils_tests/test_address_of.py

pystencils_tests/test_blocking.py

pystencils_tests/test_buffer.py

pystencils_tests/test_cuda_known_functions.py

pystencils_tests/test_custom_backends.py

pystencils_tests/test_dot_printer.ipynb