Rotation operator for freeslip

This is to add a rotation operator similar to the projection operator we already have. A test is also added currently.

The operator is capable of rotating a vector FE function on the boundaries and transforming it into its radial and tangential components, (u_x, u_y) to (u_t, u_r) in 2D and (u_x, u_y, u_z) to (u_{\phi}, u_{\theta}, u_r) in 3D.

In 3D, theoretically, there are infinite ways to choose the tangential components, but here it is done as in [Engelmann '82] which seems to circumvent the pole problem also.

I might have to do a rebase before merge.