Unit test: map between ESGF terms and STAC terms using "doctype"

[agstephens]

Overview

STAC talks about items and assets. These are equivalent to datasets and files in ESGF.
The client should allow the user to specify whether they are searching on items or assets by using: doctype=(item|asset)
And we should allow aliases for ESGF users, who can specify: doctype=(dataset|file)

Specific test requirements

1. client.search(doctype="item") should return items, so it should be identical to client.search() (because item is the default)
2. client.search(doctype="dataset") should behave the same as (1).
3. client.search(doctype="asset") should re-route to a call to client.asset_search() and should return assets.
4. client.search(doctype="file") should behave the same as (3).
5. If doctype is not in the known list: ["item", "asset", "dataset", "file"] then the client should raise an Exception that explains the error.