json convenience functions

[tomvanmele]

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[gonzalocasas]

Generally lgtm, except one question about the input and the need for some unit tests (besides the doctests)

[beverlylytle]

If the plan is to fix up all of the file readers so that they take file-like objects and paths, I think these lines of code will be used quite a few times. Maybe there should be a utility function which does this. Some sort of context manager like

class open_file(object):
    def __init__(self, fp, *args, **kwargs):
        if hasattr(fp, 'write'):
            self.f = fp
        else:
            self.f = open(fp, *args, **kwargs)

    def __enter__(self):
        return self.f

    def __close__(self):
        self.f.close()
        # but maybe you don't always want to close file-like objects...

And I suppose this could be generalized to handle reading as well.

[tomvanmele]

@gonzalocasas @Licini @beverlylytle tests pass now. can we merge this? or do we need more?