Fix bug in prior covariancematrix

In the case nbg_side=1, the prior variance of the bg was sigma_bg**2*cor_bg instead of just sigma_bg**2

da/statevectors/statevector_wrfchem.py

@@ -380,13 +380,14 @@ class WRFChemStateVector(StateVector):
                 covariancematrix[-self.nbgparams+nbg,
                                  -self.nbgparams+nbg] = \
                                      float(dacycle.dasystem["sigma_bg"])**2
-                nbg1 = (nbg+1)%self.nbgparams
-                covariancematrix[-self.nbgparams+nbg,
-                                 -self.nbgparams+nbg1] = \
-                covariancematrix[-self.nbgparams+nbg1,
-                                 -self.nbgparams+nbg] = \
-                        float(dacycle.dasystem["sigma_bg"])**2 * \
-                        float(dacycle.dasystem["cor_bg"])
+                if self.nbgparams>1:
+                    nbg1 = (nbg+1)%self.nbgparams
+                    covariancematrix[-self.nbgparams+nbg,
+                                     -self.nbgparams+nbg1] = \
+                    covariancematrix[-self.nbgparams+nbg1,
+                                     -self.nbgparams+nbg] = \
+                            float(dacycle.dasystem["sigma_bg"])**2 * \
+                            float(dacycle.dasystem["cor_bg"])
 
         return covariancematrix