Merge branch 'master-UnitTest' of https://mmg-gitlab.fjfi.cvut.cz/gitlab/jakubec/UnstructuredMesh into UnitTest-clear

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MeshDataContainer operator[] for integer sequence
JSONLogger accepts references instead of values

fix hexahedron reader

add makeBoundaryIndex

methods isInvalidIndex, isBoundaryIndex, extractBoundaryIndex

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.