Add more objects for defining searches

[dchaddock]

For all searches, we can add some more objects to help define the search parameters. This would include:

• ground
• space
• events
• max_distances
• response_format? (might become too unmaintainable, but we'll see when we get to it)
• metadata_filters

This can wait for the 0.10.0 release since I don't think there's time to make it into 0.9.0. This may change though...

[dchaddock]

When we do this, we'll want to keep the current types allowed (ie. List[Dict] for ground) to keep backwards compatibility.