Add tests for mesh data

[greschd]

Add tests for the mesh, elemental_data and nodal_data attributes on the
top-level Model, and tests for elemental_data and nodal_data on the
modeling ply.

Since the code in use is mostly the same between mesh data attributes on
different tree objects (apart from defining which elemental and nodal
data attributes are available), I think it's not really necessary to add tests
for all other objects.