Remove valley width from perturbation analysis to analytical solution

Should really revisit this approach; could be useful.
(Thanks erstwhile next-door physicist!)

grlp/grlp.py

@@ -434,7 +434,7 @@ def analytical_threshold_width(self, P_xQ=None, x0=None, x1=None,
         self.zanalytical = self.k_a * self.x**self.P_a + self.c_a
         return self.zanalytical
 
-    def analytical_threshold_width_perturbation(self, P_xB=None, P_xQ=None, x0=None, x1=None,
+    def analytical_threshold_width_perturbation(self, P_xQ=None, x0=None, x1=None,
                                    z0=None, z1=None, U=None):
         if x0 is None:
             x0 = self.x[0]
@@ -446,20 +446,18 @@ def analytical_threshold_width_perturbation(self, P_xB=None, P_xQ=None, x0=None,
             z1 = self.z[-1]
         if P_xQ is None:
             P_xQ = self.P_xQ
-        if P_xB is None:
-            P_xB = self.P_xB
         if U is None:
             U = self.U
         # Base state coefficients (no perturbation)
-        #self.P_a = 1 + 6*(P_xB - P_xQ)/7. # beta
+        #self.P_a = 1 - 6*P_xB/7. # beta
         #self.k_a = (z1 - z0)/(x1**self.P_a - x0**self.P_a) # alpha
         #self.c_a = z0 - x0**self.P_a/(x1**self.P_a - x0**self.P_a) * (z1 - z0) # gamma
         # Coefficients
         K = self.k_Qs * self.sinuosity * self.intermittency \
             / (1 - self.lambda_p) \
             * abs(self.k_a * self.P_a)**(1/6.) \
             * self.k_xQ / self.k_xB
-        P = self.P_xQ - self.P_xB + (self.P_xB - 1.)/6.
+        P = self.P_xQ
         print(P)
         # Constants of integration
         #c1 = self.U * (x0**(P+2) - x1**(P+2)) / (K*(P-2)*(self.P_a + P - 2) \