chore(lower-case): read and write cluster and bc groups in lower case

[MartinBelthle]

No description provided.

[sylvlecl]

A few comments: for the low level parsing, I think we should have a more configurable method, in particular so that we can re-use it later for the "int to string" issue, but also to have a better separation of concerns: the parsing function should not know by itself how to parse particular fields, it shuld be an argument of the function.

[sylvlecl]

Here we inject some knowledge (group should be lowercase) about specific subclasses of this class, it's not a very good separation of responsibilities.

A better design would be to specify as arguments of the method how stuff should be read or written.
For example, we could have a dict:

parsing_methods = {
    (None, "group"): lambda value: str(value).lower()
}

Meaning that all values associated to any key at the 1st level (None) and "group" key at the 2nd level should use the provided parsing method.

Actually this should probably be something we provide to he ini reader.

This would allow, in the future, to also fix how we parse integers, because we will be able to specify that we want to parse them as strings, instead of letting the reader try to guess the type.

[sylvlecl]

same comments, we could provide the serializers as arguments

[MartinBelthle]

I agree with you for the get.
But IMO the only save we'll ever need is this one and it's already generic.
So I think i should just rename the method and change the comments i wrote inside but keep the code as it is.
I'll also have to call the get with the right parsing method.
I'll commit this change and you can give me your opinion on it