Feature/update decisions

[martynpclark]

Make sure all the relevant boxes are checked (and only check the box if you actually completed the step):

• [ ] closes #xxx (identify the issue associated with this PR)
• [ ] tests passed
• [ ] new tests added
• [ ] science test figures
• [ ] checked that the new code conforms to the SUMMA coding conventions
• [ ] ReleaseNotes entry

[martynpclark]

The model decisions for summa included an option for num_method which was never used. Many existing settings files have the option itertive which is the home-grown backward Euler solution over long time steps.

The code has changed to give num_method two options: bEuler (the traditional home-grown backward Euler solution over long time steps) and sundials (the sundials solver, which will be incorporated in the code base in the near future). We have retained the option itertive (set to bEuler) so that existing settings files will be valid.

[arbennett]

@martynpclark this looks good, but I have one request - could you add a quick entry to the docs/whats-new.md list under the header Develop from your previous PR?

[martynpclark]

No need to add whatsnew because just a placeholder for the sundials solver