Added methods to return derivatives of slowness with respect to lat, lon and depth.
Modified SlbmInterface methods setGridData() and setActiveNodeData() so that they now include depth as an argument. This means that it is now possible to change the depths of the tops of the interfaces in a model.
In the Fortran interface, all methods used to be mixed case but have all been changed to lower case. This was required for the port to Linux.
Modified method GreatCircle_Xn::mnbrak() to hopefully prevent infinite loop condition.