move the math functions to a separate file; remove dependency on GSL due to license

src/gbm_mc.h

@@ -1,6 +1,8 @@
 #ifndef GBM_MC
 #define GBM_MC
 
+#include <time.h>
+
 double gbm_simulation(double spot, double rfr, double vol, double tte, double rand);
 
 double gbm(double spot, double rfr, double vol, double strike, struct tm expiry, struct tm value, int type, int sims);