Add comparisons for 3dvar_cf

[mranst]

Description

This PR enables comparison tests for 3dvar_cf, same as the other variational suites in swell.

To create a comparison suite, create an override yaml containing at least the following entry:

comparison_experiment_paths:
 - /path/to/3dvar_cf_exp1/experiment.yaml
 - /path/to/3dvar_cf_exp2/experiment.yaml

(these can be the same paths for testing purposes)

then launch swell create compare_variational_cf -o <override.yaml>

The comparison suite checks the jedi log to ensure the Residual norms are identical between the two experiments. If they are not, the suite will run eva tasks generating plots of comparisons/differences. If you don't have a test case with differing residual norms, you can check out the plots by removing the fail condition for those tasks from the flow.cylc graph