allow single-param plot with np.atleast_2d(axes)

@GregAshton this is to resolve TypeError: 'AxesSubplot' object is not subscriptable when calling with a single-parameter array. This small fix seems to work for me in PyFstat examples without changes needed for this special case anywhere else in the code, it just skips the 2D contours and returns a single histogram as makes sense:

gridcorner.py

@@ -56,6 +56,7 @@ def _get_fig_and_axes(ndim, factor, whspace):
     plotdim = factor * ndim + factor * (ndim - 1.) * whspace
     dim = lbdim + plotdim + trdim
     fig, axes = plt.subplots(ndim, ndim, figsize=(dim, dim))
+    axes = np.atleast_2d(axes) # allow single-parameter plots
 
     # Format the figure.
     lb = lbdim / dim