Check that initial conditions are continuous fields

[juliasloan25]

All tendencies must be continuous fields. We ensure this in the horizontal by calling DSS in each model at each timestep, and we ensure this in the vertical by providing continuous inputs. Since we don't have a function that can check if an input field is continuous, we should dss each of the initial states to make sure our initial conditions produce continuous fields.

See Simon's comment here for more details.

Addressed by #442