Bugfix release v1.2.0: inconsistency in differential area computation in reference state calculation

Important: This release fix a bug in computing the reference states #102 : the differential area to compute reference states, $a^2 \cos\phi \Delta\phi \Delta\lambda$ , was miscomputed as $a^2 \cos\phi (\Delta\phi)^2$ . This has not caused issues in results presented in our previous publications because we have been using ERA5 datasets with $\Delta\phi = \Delta\lambda$. Great thanks to Pragallva Barpanda for fixing the issue and also sharing her working example to preprocess model output for consumption of QGField downstream.

Summary of changes:

1. $\Delta\phi$ and $\Delta\lambda$ are precomputed in QGField object and passed into the F2PY modules as parameters
2. Pragallva has included an example in scripts/pre_process_data_in_hybrid_coordinates/ how to preprocess climate model output in hybrid coordinate for consumption of QGField downstream by (1) transforming the fields to pressure coordinates, and (2) do gridfill over underground" gridpoints using poisson solver (as how Clare did in MDTF project)
3. Added CONTRIBUTING.md to include instructions for potential contributors.