Update Python bindings

Henrik Stolzmannrequested to merge
henrik.stolzmann/dune-functions:functions-python-bindings into master

Moved the corresponding files from dune-pyreleases/2.7 to ´dune-functions` and resolved the remaining issues I encountered.

MR 189 in dune-localfunctions is important for this MR.

Edited by Henrik Stolzmann

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