The called getPropertyVector<T>() did not return
a const reference optional, but an optional<T&>.

With boost 1.61 this conversion is no longer possible.

Fixes #1383

This makes it possible to read and write std::size_t under linux and windows.

# Conflicts:
#	MeshLib/MeshEditing/Mesh2MeshPropertyInterpolation.cpp

# Conflicts:
#	MeshLib/MeshEditing/Mesh2MeshPropertyInterpolation.cpp

[U/SimpleMeshCreation] Refactoring in tool.