fix siconca variable in EC-Earth3 and EC-Earth3-Veg models in amip simulation

[egalytska]

Before you start, please read our contribution guidelines.

Tasks

• Create an issue to discuss what you are going to do, if you haven't done so already (and add the link at the bottom)
• This pull request has a descriptive title that can be used in a changelog
• Add unit tests
• Public functions should have a numpy-style docstring so they appear properly in the API documentation. For all other functions a one line docstring is sufficient.
• Circle/CI tests pass. Status can be seen below your pull request. If the tests are failing, click the link to find out why.
• Codacy code quality checks pass. Status can be seen below your pull request. If there is an error, click the link to find out why. If you suspect Codacy may be wrong, please ask by commenting.

If you need help with any of the tasks above, please do not hesitate to ask by commenting in the issue or pull request.

---

Closes #703

[egalytska]

@valeriupredoi could you please have a look at new changes that include a suggestion of @bouweandela
Btw, Codacy is still complaining =/

[valeriupredoi]

@egalytska - done and I added two tests for your fixes. I think this is ready to go 👍

[egalytska]

@egalytska - done and I added two tests for your fixes. I think this is ready to go

@valeriupredoi Thanks a lot!!