mnw2 missing itmp when zero (#742)

Resolves issue where mnw2 itmp list didn't include zeros for when a stress period had no discharge assigned. The "continue" advanced to next iteration, missing the line where zero would be added to the list. This was causing issues when writing the mnw2 file.

flopy/modflow/mfmnw2.py

@@ -1270,7 +1270,7 @@ def load(f, model, nper=None, gwt=False, nsol=1, ext_unit_dict=None):
                                        hlim, qcut, qfrcmn, qfrcmx] + xyz)
                 stress_period_data[per] = current_4
             elif itmp_per == 0:  # no active mnws this stress period
-                continue
+                pass
             else:
                 # copy pumping rates from previous stress period
                 mnw[wellid].stress_period_data[per] = \