General reductions on host

See merge request mmg/tnl-dev!7

According to benchmarks, there is practically no difference in
performance. Only explicit unrolling is helpful, but that has been
implemented for the general algorithm in Reduction::reduce as well.

Now that error handling is done via exceptions, the return value is free
for more sensible things.

Refactoring: changed return types from bool to void

See merge request mmg/tnl-dev!6

[skip ci]

The exception is not used anywhere and does not make much sense.
Functions/methods which are not implemented yet can just throw
std::runtime_error wich a more specific message.

See https://jlk.fjfi.cvut.cz/gitlab/mmg/tnl-legacy

The method was either returning true or throwing an exception, so it can
simply return void or throw an exception.

(Jak má člověk propalovat čas v Ostravě, když mu to školitel rozbil....
git log -L60,60:src/TNL/Meshes/GridDetails/Traverser_Grid3D_impl.h)

* mpi:
  Fix DistributedIOTest

* cineca/ci:
  Added configurations to build with MPI on the CI
  Disabled building Benchmarks, Examples, Tools and Python in one half of the CI jobs
  Reorganization - removed useless subdirectory
  Reorganization - moved examples into src
  Reorganization of benchmarks - moved into src