Consider programmatically generating `methods.R`

[Bisaloo]

This was suggested by Vince:

We currently have a tension where the methods.R file is either very long and extra verbose, or more concise but not very easy to understand.

One potential way around this would be to have a very verbose methods.R which is programmatically generated. This way, the repetition in this file is no longer a problem because we would only edit the file to generate it.

Some additional references:

• https://blog.r-hub.io/2020/02/10/code-generation/
• this experimental package, where all the code in R/ is programmatically generated: https://github.com/bisaloo/signaturechecker