Loops

• improved documentation of ParallelFor
• renamed StaticFor to unrolledFor (reimplemented as a free function) and also improved its documentation
• reimplemented TemplateStaticFor with constexpr functions and generic lambdas
  • interface renamed from TemplateStaticFor to staticFor
  • also added tests and updated documentation, read it for details
• implemented tests for checking the consistency between Mesh and all of its entities