Add support for string coordinates

[jvegreg]

To solve the issue raised by @ledm in ESMValGroup/ESMValTool#1621

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.

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[valeriupredoi]

looks good to me, cheers @jvegasbsc

[valeriupredoi]

reran the test container that failed with a segfault coz of bleh ye olde hdf5

[mattiarighi]

Looks fine to me, but @ledm should confirm whether this solves his issue.

[sloosvel]

I tried this with data that contains string coordinates and it worked, thanks @jvegasbsc

[bouweandela]

Thanks everyone! Merging this now, since today is the deadline for being included in the first version 2 release.