CLN FloatArrayFormatter
I'm working on #12164

This is a work in progress. 

Ok, that should do.

What I have done:
- Instead of differentiating on the one hand `FloatArrayFormatter.get_result` (which comes from `GenericArrayFormatter`), and on the other hand `FloatArrayFormatter.get_formated_data`, according to their destination (two usages, two functions, two independent pieces of code), I differentiate with the type of the output: `.get_result` keeps returning a list like before, and the new `.get_result_as_array` returns a numpy array like `.get_formatted_data` used to.
- The question "do we want a fixed-width output?", which is a formatting question, is translated like all formatting matters into a parameter `fixed_width` for the `GenericArrayFormatter` constructor (boolean, default `True`, because that's the main usage as of today)

This seems (to me) the simplest way to know which method should be used.

I have merged the two independent pieces of code that we used to have, and which were doing basically the same thing. But `FloatArrayFormatter.get_formated_data` was more refined, as it was using a mask to deal separately with the NaN values. I've kept that approach, but I don't know if it is the best option in term of performances.
 All green
 this looked ok last time. pls rebase and repush.
 @jreback all set
 does this actually change any features? e.g. implement the decimal fix? (or is that another PR), this just cleanup?
 thanks!
 No, that was just a cleanup, that's why I haven't touched the tests. Now I should be able to move on to the decimal parameter (issue #12031)
