fixes #69

Before this change, the error was:

XMLVTK error: failed to parse the file as an XML document.

fixes #70

Loops

See merge request !94

The kernels are not interesting for users and Doxygen made a mess out of
various CUDA declarations. The 'detail' namespace is considered internal
and excluded from processing by Doxygen.

- passing indices and array/vector values by reference is useless
- declaring arguments passed by value as "const" is useless
- using "IndexType" and "ValueType" is better than "Index" and "Value",
  because Doxygen creates an automatic link to the description of the
  type
- various other documentation and whitespace fixes

It is an implementation detail specific to the CutMeshFunction class,
not something that should be part of the public interface. There are
many problems with this approach: e.g. it does not scale to higher
dimensions and it is not parallel.

- interface renamed from `TemplateStaticFor` to `staticFor`
- also added tests and updated documentation, read it for details

The main purpose of this algorithm is to do loop unrolling, not to
handle static/constant bounds. The true StaticFor will handle the
iteration indices in such a way that they can be used in constant
expressions (e.g. passed as template arguments).

Mesh tutorial and documentation changes and some refactoring

See merge request !92

The new value is not correct, it breaks e.g. ParallelFor2D and
ParallelFor3D (for which we had disabled tests by mistake). We need to
separate the max values for x, y and z axes, see the linked Gitlab
issue.

This reverts the commit 37632fd9

The Experimental directory does not make much sense - there is a lot of
experimental or just unused code in TNL, e.g. Images or Operators.

- tests moved to the main UnitTests directory
- added missing inclusion of the Algorithms tests directory to
  CMakeLists.txt