fix of operator- for traited classes

possible use of ADL.

const getter).

To achieve the maximal efficiency of traits approach is necessary to
change name type from std::string to const char*.
Moreover, I had to make the default trait public in another way. There
is a function creating and returning the generated Trait. At first sight
is seemed to me as bad idea, but the compiler can not work so
efficiently with it. Thus, it is better to have static function which
generates the object of traits.

constexpr reference like cls.attr.
The tellg does not work as expected in text mode. There is problem with
CRLF on windows. Thus, the file must be opened in binary mode.

MemberReference.

traits apply

way how to detect whether the mesh changed for example for export
purpose.

MeshDataContainer operator=(& and &&) and little documentation

fix of methods setting up the boundary cells

This reverts commit a6c71d46, reversing
changes made to 0a524e29.

Refactor

performance.