Update process script to recursively search with met sub-directories

[rt17603]

Original report by Rachel Tunnicliffe (Bitbucket: rt17603, GitHub: rt17603).

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For the met files associated with a NAME run, this is sometimes organised into sub-folders (e.g. split by year). When running the process script this should be able to search the Met folder and any sub-folders (recursively) to search for a suitable met file to read.