Create figure in `plot_distr_funct` if ax is None

Create a new figure in function `plot_distr_funct` if ax is None.

Inst_eddies/Analysis/distribution_function.py

@@ -13,7 +13,7 @@ def plot_distr_funct(x, label = None, ax = None):
     nx = np.size(x)
     
     if nx != 0:
-        if ax is None: ax = plt.gca()
+        if ax is None: fig, ax = plt.subplots()
         ax.plot(x, (1 + np.arange(nx)) / nx, label = label)
         print("minimum value:", x[0])
         print("maximum value:", x[-1])