Added fixes for tos of BCC models

[schlunma]

This PR adds fixes for tos of the CMIP6 BCC models

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.
• If writing a new/modified preprocessor function, please update the documentation
• 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.
• Please use yamllint to check that your YAML files do not contain mistakes
• If you make backward incompatible changes to the recipe format, make a new pull request in the ESMValTool repository and add the link below

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

---

Related to #537. (Note: Automatic closing of these issues is not desired; they should be closed when the issue is reported to the modelers).

[npgillett]

@schlunma This fix should also be applied to siconc - I got the same error as above for tos, when I tried to process siconc from BCC-CSM2-MR. This was also raised in #151. Thanks...

[schlunma]

This probably needs to be applied to many ocean variables. Unfortunately, I did/do not have the resources to check all of them, but you can simply use this line as a template on how to easily apply an already present fix to another variable (an another model if necessary). In your case this would be something like Siconc = BaseTos.