Changed manner in which derivatives of tt with respect to lat, lon are computed. Removed all derivatives of slowness from all interfaces.

Fixed bug related to computing derivative of tt with respect to depth that happened when depth step spanned the Moho.

Changed default step size used to compute horizontal derivatives from .01 to .001 radians. Changed default step size used to compute depth derivatives from 0.01 km to 0.1 km. Added getters and setters for del_distance and del_depth, which are the step sizes for calculating horizontal and depth derivatives.

## Added methods:

- getDistAz() – distance and azimuth from A to B
- movePoint() – from A, move distance azimuth to B
- getPiercePointSource()
- getPiercePointReceiver()
- getGreatCircleLocations() – get points along headwave interface
- getGreatCirclePoints() – get nPoints between A and B
- getGreatCirclePointsOnCenters() – get nPoints between A and B

In the C and Fortran interfaces, error codes returned by all the methods have been changed. The methods used to return 0 indicating no error, or 1 indicating an error occurred. The methods still return 0 when no error, but now return positive, non-zero error codes for error conditions.

Replaced all the test programs with new ones that perform a simple regression test. These new test programs are better examples of how to use SLBM.