Optimized mesh initialization by skipping entities which are completely disabled in the configuration

Added options to force the output RealType and GlobalIndexType in tnl-mesh-converter and tnl-refine-mesh

The example should be the same for both .cpp and .cu, otherwise the
output might be inconsistent (e.g. when .cpp is included, but .cu is
compiled and its output is used).

Fixed Jacobi and SOR so that code using getLinearSolver() and a distributed matrix can be compiled (even if the solvers do not work...)

Added dummy methods reduceRows, reduceAllRows, forElements, forAllElements, forRows, forAllRows to DistributedMatrix

The IsArrayType trait does not include TNL::String...

Polygonal + polyhedral mesh

See merge request !93

It was broken because the MeshBuilder interface was changed due to
polygonal and polyhedral meshes, but tnl-decompose-mesh was not built on
the CI due to a missing optional dependency (metis).

See MR !107

- refactored a few for cycles in EntityInitializer to use ParallelFor
- modified function findEntitySeedIndex in Initializer to use find instead of insert
- modified FPMAReaderTest to use slightly bigger mesh for testing
- added a few explanatory comments
- deleted unused methods from SubentitySeedsCreator
- created specialization of Formatter in Assert.h for std::pair to avoid compilation error caused by calling inplaceExclusiveScan on array of pairs

Added EntityInitializerLayer specialization for initializing polyhedron subentites 3 -> 2 from cell seeds + other minor mesh refactoring