Move MeshSubsets related code to MeshLib.

Rename MeshItem.h to Location.h.
Move MeshLib::Location related implementations into cpp file.
In MeshSubsets remove not required 'explicit' keyword.
Test MeshSubsets meshes uniqueness.
Add MeshSubsets meshes uniqueness precondition in ctor.
Add MeshSubset::getNodeID().
Correct return value of MeshSubsets::getNMeshes().