Add a page listing how to do some common operations.

[sebcrozet]

See https://discourse.nphysics.org/t/documentation-about-calculations-and-arithmetics/387 for a detailed comment.

This page would list the kind of operations like the_matrix.column_iter_mut().for_each(|mut col| col += &the_vector) that can be very useful but don't have a dedicated method.