Deep testing of external variables #99
Description
The motivation
As a user, if a data file says that there external variables, it would be good to check they are actually there.
The proposed solution
In the validate tree API, if a file is found that says it has external variables, check a) that the external variables can be found and b) that they line up with the data.
Alternatives
Additional context
The CF-conventions provide some rules for this (https://cfconventions.org/Data/cf-conventions/cf-conventions-1.12/cf-conventions.html#external-variables). We should use this, and build the things that CF-conventions doesn't do on top (e.g. CF-conventions doesn't specify how to find the external variables, but we have a DRS so we can do that check).
This is the leftover piece of #98.