Breaking changes in v4 release

[jpswinski]

We are planning to release v4.0.0 of SlideRule in the next few days. It is a major version update because it includes a handful of breaking changes. We want to make sure everyone is aware of these changes to the greatest extent possible. They will be detailed in the release notes which can be found here: release-v4.0.0.md. In summary:

• We've standardized the fields returned from ICESat-2 based endpoints so that they all (if applicable) have the following fields: time, longitude, latitude, x_atc, y_atc. This simplifies downstream processing code (and we also believe is easier for end users). But it required us to change the name of some of the atl03 and atl06 fields.

• We've removed the asset parameter from the icesat2 Python module APIs. The Python client now handles selecting the best asset given the user's request. Users can still override which asset is used by supplying it in their request parameters structure, but we believe requiring the user to know which asset to use was confusing and unnecessary.

• We've reworked the way ancillary data is handled by both the server and the client when ancillary fields are requested. This change doesn't manifest itself directly to users but it does mean that everyone will need to update their Python client to the latest version before being able to use the new deployment of SlideRule once we release v4.0.0.