Cleaning FloatArrayFormatter
While working on issue #12031 (adding a `decimal` parameter to `.to_latex()` and `.to_html()`), I realized a solution would be pretty straight-forward if only `FloatArrayFormatter` was a bit cleaned.

Currently, there are two methods that do a similar work:
- `.get_formatted_data()`, which returns the array with its float values converted to strings using the parameters given at initialisation.
- `.get_result()`, which does something similar, but with a fixed-width output suitable for screen printing. 

The two methods don't share any piece of code.

(The first method can already deal with a `decimal` parameter. The second cannot, and it is the one used by `.to_latex`.)

This is in part my fault, since my pull request #11681 could have been more factorized.

Anyway, I'm volunteering to do a PR.
