feat: provide a way for Form.select_columns to distinguish structural dots from dots in the names of record fields

[tcawlfield]

Addressing the need to select field names with literal dots in them.

This is a change to Form.select_columns(). This change allows a user to supply a specifier as a list of lists of field (or column) names.

[jpivarski]

This looks great!

The one error doesn't seem to be related to this, and it's persistent enough to survive re-running the worker. (It looked to me like an installation/packaging issue, so that's why I tried re-running.)

E       AssertionError: Regex pattern did not match.
E        Regex: "Can't pickle"
E        Input: "Can't get local object 'test_serialise_with_nonserialisable_attrs.<locals>.<lambda>'"

The second error message does seem to be related to not being able to pickle; lambda functions generally aren't pickleable. But somehow the "Can't pickle" message isn't there.

[jpivarski]

Selecting everything that matches, when there's ambiguity, is a good choice.

Thanks for the detailed docstring on column selector syntax!

I just have a few comments on the error messages themselves, and broadening the argument to accept any iterable, not just lists and tuples. (It's not unusual for a Python user to not even know the exact type of collection they're using, just that it's a collection. For instance, it could be an array, a set, an iterator from some_dict.values() (which is not a list), etc.)

[jpivarski]

Also, I've updated the branch, so be sure to git pull if you're editing a local copy (instead of just patching in small fixes through the web interface).