Added Ram-Lak and SheppLogan window function in Ramp filter.

Dear Simon Rit,
I currently using RTK for reconstruction of industrial CT data. I add Ram-Lak and Shepp-Logan to the Ramp filter. I was considering to separate the ramp filter class and window function class (to easily add new/custom window function in the future), but it will affects the current API, so I end up with this solution. Please consider to merge these changes.

best regards,
P.S. I think I met you during ICCR 2013 in Melbourne.