Add Counter struct.

This can be used for counting of meshes or other object instantiations etc.
Use BaseLib::Counter<Mesh> for mesh id.
Add UniqueMeshId test.
Merge pull request #98 from endJunction/AddMeshId

Add mesh ID.
Adding std::array ctors to TemplateXXX.

Using std::array is safe, so users should prefer this constructor to the older constructor with pointers.