fix: readme and static const DT

advection/src/AdvectionKernel.h

@@ -5,22 +5,23 @@
 
 #include "Vel.h"
 
-#define DT 50
 
 /*
  * Implement this class for every integration method.
  */
 class AdvectionKernel {
 public:
+    const static int DT = 50;
     /**
      * This function must take a time, latitude and longitude of a particle and must output
-     * a new latitude and longitude after being advected once for DT time as defined above.
+     * a new latitude and longitude after being advected once for AdvectionKernel::DT time as defined above.
      * @param time Time since the beginning of the data
      * @param latitude Latitude of particle
      * @param longitude Longitude of particle
      * @return A pair of latitude and longitude of particle.
      */
     virtual std::pair<double, double> advect(int time, double latitude, double longitude) const = 0;
+
 };
 
 #endif //ADVECTION_ADVECTIONKERNEL_H