add pskernelfunction and parameter collector

pystencils/nbackend/ast/__init__.py

pystencils/nbackend/ast/analysis.py

pystencils/nbackend/ast/kernelfunction.py

pystencils/nbackend/ast/nodes.py

pystencils/nbackend/ast/util.py

pystencils/nbackend/c_printer.py

pystencils/nbackend/exceptions.py