Method to retrospectively read external variables

External variables can currently be read-in using the external keyword argument to cf.read, but not at a later stage. So that users can read any external variables after they have read in the corresponding fields, with the external variables only referenced as an external_variables attribute, we decided we would like a method on a Field that can retrospectively find and read-in the relevant constructs from the referenced external files.

The motivation is the case that the user has already manipulated the field when they realise they want to read the external variable(s); this method would save them from having to re-read and re-process.

The name should be considered further at the time of implementation, but something like Field.get_external_vars() or Field.read_external_vars() is illustrative.

Start by tracing the existing cf.read handling of the external keyword and the Field representation of the external_variables attribute. Define and implement a Field method that finds and reads the relevant constructs from referenced external files after the field has already been read and manipulated; done means users no longer need to re-read and re-process the field.