[WIP] Python bindings

[ftalbrecht]

This introduces python bindings based on pybind11, which comes as a new optional dependency (dune-pybindxi).Things we might want to change (though not necessarily in this pr):

• Bindings are unconditionally built if dune-pybindxi is found. There should be a define which defaults to true but allows the user to disable bindings.
• There are no tests. We should at least test that we can import the python extension module without errors.

Before the merge we also need to revert fb419887ab3ce91d21e9263d2d8e76df988e1521.

[codecov-io]

Codecov Report

Merging #24 into releases/2.4 will decrease coverage by -0.19%. The diff coverage is 100%.

@@               Coverage Diff                @@
##           releases/2.4      #24      +/-   ##
================================================
- Coverage         61.46%   61.27%   -0.19%     
================================================
  Files                70       70              
  Lines              6311     6275      -36     
================================================
- Hits               3879     3845      -34     
+ Misses             2432     2430       -2

Impacted Files	Coverage Δ
dune/xt/common/fmatrix.hh	88.88% <ø> (ø)	:white_check_mark:
dune/xt/common/string_internal.hh	78.14% <ø> (-0.15%)	:x:
dune/xt/common/configuration.hh	84% <ø> (-2%)	:x:
dune/xt/common/memory.hh	90.24% <ø> (ø)	:white_check_mark:
dune/xt/common/timings.cc	86.29% <100%> (-1.02%)	:x:
dune/xt/common/timedlogging.hh	100% <100%> (ø)	:white_check_mark:
dune/xt/common/test/main.hxx	62.5% <ø> (-1.5%)	:x:
dune/xt/common/configuration.cc	51.13% <ø> (-0.87%)	:x:
... and 13 more

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[ftalbrecht]

Done in f6f1b337337778ab5894bdbd8050d2058935f309.