Fix/compact form integrators
This commit fixes the 3rd and 4th order Adams-Bashforth methods and used the compact form for the Butcher tableau. There are no tests for these methods. I also added a comment about using the compact form of the Butcher tableau for all three methods. This comment was also added to the Forward Euler.
I also did some housekeeping and removed the unused first order skeleton code for the Adams Moulton method which is the backward Euler.