FieldCoarsenOperatorTest in 3D

[nicolasaunai]

FieldCoarsenOperatorTest needs to be implemented in 3D

[PhilipDeegan]

does your magnetic flux PR affect this @nicolasaunai ?

[nicolasaunai]

The MagneticFieldCoarsener has :

 else if constexpr (dimension == 3)
         {
             throw std::runtime_error("Not Implemented yet");
         }

the DefaultFieldCoarsener (former FieldCoarsener is 3D, however test_coarsen_field has. for ndim in [1, 2]:)

[nicolasaunai]

done as part of #960