Fixed some bugs having to do with the valid range of the Pn travel time for mantle events.

Changed the algorithm for computing Pg/Lg travel times. Code now returns Pg/Lg travel times for events at any depth in the crust, even events below the top of the middle crust.

Added new SLBMInterface constructors that take an earth radius in km. When the constructors with no parameters are called, the radius of the earth is assumed to vary as a function of latitude, as described by a GRS80 ellipsoid. When the earth radius is specified in the SLBMInterface constructor, then earth radius is assumed constant at the specified value. Geographic latitudes are still converted to geocentric latitudes, using GRS80 ellipsoid.

Modified the IASPI91_SLBM.txt model file to have 9 layers instead of 8.