[cmake/py] relocate all bindings related code

CMakeLists.txt

dune/xt/functions/CMakeLists.txt

dune/xt/functions/ESV2007.hh

dune/xt/functions/affine.hh

dune/xt/functions/checkerboard.hh

dune/xt/functions/combined.hh

dune/xt/functions/constant.hh

dune/xt/functions/default.hh

dune/xt/functions/derived.hh

dune/xt/functions/indicator.hh

dune/xt/functions/interfaces.lib.hh

dune/xt/functions/random_ellipsoids.hh

dune/xt/functions/reinterpret.hh

python/CMakeLists.txt

python/dune/xt/functions/CMakeLists.txt

python/dune/xt/functions/ESV2007.bindings.hh

python/dune/xt/functions/bindings.cc

python/dune/xt/functions/checkerboard.pbh

python/dune/xt/functions/constant.pbh

python/dune/xt/functions/expression.pbh