Add the ability to attach datasets to meshes.

Introduce smtk::mesh::DataSet, which provides the ability to assign to the cells of meshsets double-valued data of user-defined tuple length. Change InterpolateMeshElevation operator to InterpolateMeshDataSet and change its functionality from modifying the z-coordinate of a mesh's points to adding interpolated data to a meshset's cells according to the mesh's centroid. Augment ImportVTKData andExportVTKData to accept and create double-valued VTK cell data, respectively.