Added Checkpoint filter.
Updated solvers to include new checkpoint filter. Updated XML format of filter input. Now pass session reader to filters. Fixed filters for parallel execution. Changed default legacy checkpoint interval to zero to allow replacement with new filter. Closes ticket #66. git-svn-id: https://gforge.sci.utah.edu/svn/nektar/trunk@3672 305cdda6-5ce1-45b3-a98d-dfc68c8b3305