'derivemxE' new name proposal

[yosakaon]

What about renaming derivemxE to deriveEjacobian ? https://github.com/math-comp/analysis/blob/14c5bf819bce88100955bc655f0307568e13391c/theories/derive.v#L356 In my opinion, it is more appropriate because we can see by the name which alternative definition of directional derivative we're about to use (by the way, deriveE could maybe become deriveEdiff. to me both are equally valid alternative definitions of directional derivatives).