Unified singleton data manager class

[ebchin]

This PR combines the CouplingSchemeManager and MeshManager into a templated DataManager class. The underlying mesh and coupling scheme data are stored in a std::unordered_map<IndexT, T>, where T = CouplingScheme or MeshData. Changes are made in source to ensure the keys sent to the unordered_map are of type IndexT, and consistency in naming the key in the source is introduced. mesh_id or mesh_id# are used for MeshManager keys and cs_id is used for CouplingSchemeManager keys.