refactor(python): deprecate columns param for DataFrame init; transitioning to schema

[alexander-beedie]

Here we go... deprecating columns in favour of schema for DataFrame init, tying in with the recent addition of schema_overrides. Can continue to use columns while we transition (which we may want to do over an extended period, given how ubiquitous it is).

• Transition is managed via the usual @deprecate_alias decorator.
• Updated external and internal interfaces.
• Updated/improved docstrings.
• Updated tests.

This can release at any time, as it's not a breaking change.

For the next breaking release, I'll make a slight param reorder so that schema and schema_overrides are adjacent in all constructor methods.

[ritchie46]

Nice! :)