Implementation of Leapfrog class of solvers

[szaghi]

This is just the roomwhere we discuss the implementation of Leapfrog class. This is in my radar for the next 2 weeks.

[szaghi]

Done in v0.0.6.

I used the more recent Williams proposal (RAW filter, see [1]), but allow also to perform unfiltered integration or standard (1st order resulting) RA filter: this is driven by the filter-coefficient values used for initializing the integrator. For performing an unfiltered integration, simply do not pass the filter argument to the integrate subroutine.

[1] The RAW filter: An improvement to the Robert–Asselin filter in semi-implicit integrations, Williams, P.D., Monthly Weather Review, vol. 139(6), pages 1996--2007, June 2011.