Refactor IO api for clarity and allow computing chunks directly on index

[glopesdev]

There was a confusingly named function called aeon remaining from the original first draft of the API. This could be confused with aeon the module name, so here we propose deprecating this time conversion function and replace it with two new explicitly named functions to_datetime and to_seconds instead.

In addition, we extend the chunk function to allow extracting chunks directly from a DatetimeIndex object.