ba_plot: add a function to export the plot

Wrap/Python/src/bornagain/ba_plot.py

@@ -466,6 +466,12 @@ def show_or_export(**plotargs):
         plt.show()
 
 
+def export(**plotargs):
+    _figfile = plotargs.pop('figfile', None)
+    if _figfile:
+        plt.savefig(_figfile, format=plotargs['saveformat'], bbox_inches='tight')
+
+
 def plot_simulation_result(result, **plotargs):
     """
     Draws simulation result and (optionally) shows the plot.