Added maxtem property to the Model class in addition to a spatial_type attribute
which tells us whether a model is currently in the plane wave or modal basis. These
attributes can be specified on construction of the Model.

The line model.homs.append("00") is no longer required for plane-wave (nor HOM)
models - the homs list is kept empty for plane-wave models whilst it is filled
with the necessary modes for modal models.

Each components' _on_build (and _fill_matrix where necessary) will now check for
whether a simulation is modal or not and set the coupling matrix sizes accordingly.