Design error handling mechanism in case Uref cannot be solved

[csyhuang]

So far when working with model/reanalysis data, there are two scenarios in which Uref cannot be solved, which would raise an error when calling QGField.compute_reference_states and the user cannot proceed to calculate LWA:

(1) The input field is unphysical/unrealistic or vertical resolution is too low that cannot capture the stratosphere structure
(2) The input field is realistic but latitudinal/vertical resolution is too high

In case (1), it is legitimate to throw an error since the input is problematic. However, in case (2), user shall be able to proceed to computation of LWA - even though Uref is not numerically solvable, LWA (but not the fluxes) can still be computed since computation only requires Qref.

This task would involve two components:

• design an error handling mechanism that the specific error "Uref not solvable" can be caught and identify properly (such that some external function would return True when "Uref not solvable" happens) when user calls QGField.compute_reference_states and still return Qref (maybe Create another class of QGField that computes only QGPV and LWA #113 is needed).
• Allow user to proceed to the computation of LWA even when Uref is not solvable. This would involve logic to trigger the implementation mentioned in Create another class of QGField that computes only QGPV and LWA #113 . To disentangle the two tasks, assume the following method of QGFieldBase exists:

def compute_lwa_only(self):
    """
    This returns layer-wise LWA(height, lat, lon) and FAWA(height, lat)
    """

[csyhuang]

This is tackled by b45c405 :
After calling QGField.compute_reference_state, if Uref cannot be solved, QGField.nonconvergent_uref will be set to True, which will be checked in the MDTF procedures.