Move Columns docs for Readers from the class docstring to the read method docstring

[jkbhagatio]

No description provided.

[lochhh]

semi-related #300

[glopesdev]

@jkbhagatio can you elaborate the proposal a little bit more? I am assuming the idea is to document the columns of the resulting dataframe returned by read?

The main reason why I left the docs at the level of the Reader class is that the read method is not intended to be accessed directly by users of the api (what they call is aeon.load with a certain Reader object).

Because the Reader class has to be definitely specified, I felt it would be more accessible to find help for the format of the data there. Furthermore, some of the readers have constructors that themselves take columns so leaving it at the class level was taking care of both levels of documentation.

[jkbhagatio]

Sorry, this issue should read "... to the constructor method docstring", not "...to the read method docstring".

One motivation was to document the constructor methods, which would also help satisfy our requirement of docstringing all our functions, and consequentially the autobuilding of docs, and consequentially I think make things clearer for those creating new Readers.

Also all classes should have an 'Attributes' section as @lochhh refers to above and as mentioned in the style guide we are following: https://google.github.io/styleguide/pyguide.html#s3.8.4-comments-in-classes

This may be somewhat moot if we use a package to write cleaner Python OOP (i.e. in this case remove boilerplate code like the constructor method) like 'attrs' or 'pydantic' as mentioned here: #233 and as discussed today on Teams.