sets used instead of lists when membership testing

[shimwell]

Description

I think this will make nearly no difference on speed and we probably don't want to worry about the speed of python code. However I think sets are slightly quicker for membership checking compared to lists. Although they might be slower to create in the first place. Perhaps it makes sense to make these constants sets as they are used in the cv.checkvalue to check the user input is in the available options.

Also I noticed one defined constant that is not used _XS_SHAPES so I've removed that

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)

[paulromano]

@shimwell In most cases here, you are changing an object from a list [a, b, c] to a tuple (a, b, c) instead of a set {a, b, c}. For some cases (e.g., _SUBSHELLS), it will definitely need to be a list or a tuple because the object is indexed. For many other cases, a set would be preferred if all it is being used for is membership tests. Can you go through this PR again and use sets where they are appropriate?

[shimwell]

Sorry about this mess of a PR, I got mixed up with suggesting tuples for their immutability as I think gives us a bit more safety that using lists and suggesting using sets defined like this set() instead of {} and I forgot to add the set and ended up going for tuples.

I shall work through these corrections and see if we index them prior to committing them