STYLE: Make Kriging hyperparameter property naming consistent

Make Kriging hyperparameter property naming consistent: use
`hyperparameter_l` instead of `hyperparameter_beta_l` for the `beta_l`
hyperparameter, much like `hyperparameter_a` is being used for the
`beta_a` hyperparameter.

src/eddymotion/model/gpr.py

@@ -288,7 +288,7 @@ def hyperparameter_a(self) -> Hyperparameter:
         return Hyperparameter("beta_a", "numeric", self.a_bounds)
 
     @property
-    def hyperparameter_beta_l(self) -> Hyperparameter:
+    def hyperparameter_l(self) -> Hyperparameter:
         return Hyperparameter("beta_l", "numeric", self.l_bounds)
 
     def __call__(
@@ -394,7 +394,7 @@ def hyperparameter_a(self) -> Hyperparameter:
         return Hyperparameter("beta_a", "numeric", self.a_bounds)
 
     @property
-    def hyperparameter_beta_l(self) -> Hyperparameter:
+    def hyperparameter_l(self) -> Hyperparameter:
         return Hyperparameter("beta_l", "numeric", self.l_bounds)
 
     def __call__(