Add warning for monthly timesteps (#1208)

* Add warning for monthly timesteps

Add warning if store_monthly_step == True but mb_elev_feedback = 'annual' to make sure users don't expect output to reflect monthly processes if mass balance is only calculated annualy

* Update flowline.py

correct for minor mistakes in warning.

oggm/core/flowline.py

@@ -2128,6 +2128,10 @@ def flowline_model_run(gdir, output_filesuffix=None, mb_model=None,
     kwargs : dict
         kwargs to pass to the FluxBasedModel instance
      """
+    mb_elev_feedback = kwargs.get('mb_elev_feedback', 'annual')
+    if store_monthly_step and (mb_elev_feedback == 'annual'):
+        warnings.warn("Mass balance is computed yearly. If you want output to "
+                      "reflect monthly processes set mb_elev_feedback = 'monthly'")
 
     if cfg.PARAMS['use_inversion_params_for_run']:
         diag = gdir.get_diagnostics()