Set dims to longitude, latitude if projection passed

[santisoler]

Make BaseGridder.grid, BaseGridder.scatter and BaseGridder.profile to use
("latitude", "longitude") as default dims if a projection is passed.

After fatiando/harmonica#233

Reminders:

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• Add tests for new features or tests that would have caught the bug that you're fixing.
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[santisoler]

We're breaking some backward compatibility by returning new set of coord in the generated outputs, that's why I had to change some tests. Is it OK or should we leave this PR for 2.0.0?

[leouieda]

Yep, probably best to wait for 2.0. We should start a branch to gather these PRs.

[leouieda]

I've changed the base here to 2.0-dev so once the merge conflict is resolved we can merge this.