Subtype of `CoupledSDEs` for large deviation theory functions

[reykboerner]

The new CoupledSDEs struct (see issue #76) allows a lot of flexibility in defining the stochastic system, including non-linear and time-dependent noise functions. Most of the large deviations functionality, however, requires a specified noise_strength and (for now, constant) covariance_matrix.

• We need to add warnings/errors so that large deviations functions such as om_action or geometric_min_action_method are not applied to CoupledSDEs systems they are not applicable to.

• Eventually, we should define a subtype of CoupledSDEs that restricts to SDEs compatible with large deviation theory, and make the relevant functions work only on that subtype.