Add accuracy parameters for geopoint

[lognaturel]

Closes #579

Why is this the best possible solution? Were any other approaches considered?

This is the approach that matches other parameters currently. I feel more and more of a desire to reduce the spaghetti nature of that part of the code but I don't think we should combine that with adding a new parameter.

What are the regression risks?

I had to combine this with allow-mock-accuracy which also applies to geopoint questions. So there's some risk there. I added test to verify that the new attributes aren't allowed with trace/shape and that all of the point parameters combine well.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

XLSForm/xlsform.github.io#222

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run nosetests and verified all tests pass
• run black pyxform tests to format code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

The goal would be to get this in a new pyxform release by Tuesday 2/15!

[lindsay-stevens]

Looks good! Just a suggestion about the warning message, but it's not really a blocker.