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

The benchmarks, examples, tools or Python bindings are not run by the CI, so
we just check that they can be compiled. But we don't have to do it twice...