Refactor slightly Snes class to handle more cleanly the different kind of solvers.

Due to a crash of the Redmine server that was used from 2013 to 2019, all the ticket history of the project circa 2019 has been lost. An effort has been made to reconstruct the most possibble (see #1606 (closed) for more details), however clearly a lot of content was lost. So:

• If the ticket is tagged as 'Redmine placeholder', it means no informations was retrieved (it might not have existed in the first place - once again have a look at #1606 (closed) for more details).
• If the content of a subsequent note is formatted as a Redmine mail informations, it means a mail sent when the issue was modified as been used to recreate at least part of it (see #1607 (closed) for more details).
• If another content is present, informations might have been filled from the Changelog.md file.