fixed "gaussiandotdot", "gaussiandotdotnorm" and "ricker" formulae to produce the correct centre frequency wavelet as requested by the user.

gprMax/waveforms.py

@@ -43,7 +43,11 @@ class Waveform:
         Returns:
             waveform (float): Calculated value for waveform.
         """
-        
+        if self.type == 'gaussiandotdot' or self.type == 'gaussiandotdotnorm' or self.type == "ricker":
+            chi = np.sqrt(2) / self.freq
+            zeta = 2 * np.pi * np.pi * self.freq * self.freq /2
+            delay = time - chi
+        else:
             chi = 1 / self.freq
             zeta = 2 * np.pi * np.pi * self.freq * self.freq
             delay = time - chi