Non-linear h operators

[kysolvik]

The problem
In theory, we support both linear and non-linear observation operators:

• Linear operators (H) must be array-like. These are what we have been using in all our examples, etc.
• Non-linear operators (h) must be callable functions. Initial support was added for these, but it hasn't been updated for a while and so likely they no longer work, at least with some of the DA methods.

The fix

1. Try using callable h operators within each DA method (3DVar, ETKF, 4DVar, 4DVar-Backprop) to see where it does and does not work.
2. Update dacycler code to make h operators functional.