Improve fullwidth env

[dbosk]

• [ ] For one thing we can block the use of the margin when we use a fullwidth figure: https://tex.stackexchange.com/questions/420099/avoid-conflict-between-marginpar-and-full-width-figure

• [x] Second: it seems that adjustwidth* doesn't check \if@twoside.

[dbosk]

Now the first point is in #15 .