fix(modflow/mfriv) unstructured mfusg RIV package load check fix

flopy fails on checking mfusg unstructured RIV packages

* add unstructured clause / disu reference to modflow/mfriv
* modify autotest/t016.py test_usg_rch_evt_models01 to include a riv file and
  check() call

autotest/t016_test.py

@@ -210,6 +210,7 @@ def test_usg_rch_evt_models01():
     m = flopy.modflow.Modflow.load(
         nam, model_ws=model_ws, version="mfusg", exe_name=v
     )
+    m.riv.check()
     m.model_ws = tpth
     m.write_input()
     if run:

examples/data/mfusg_test/rch_evt_tests/usg_rch_evt.nam

@@ -5,6 +5,7 @@ DISU  29 usg_rch_evt.disu
 OC  22 usg_rch_evt.oc
 RCH  18 usg_rch_evt.rch
 evt  17 usg_rch_evt.evt
+riv  16 usg_rch_evt.riv
 CHD  199 usg_rch_evt.chd
 LPF 11 usg_rch_evt.lpf
 DATA(BINARY) 50 usg_rch_evt.cbb

examples/data/mfusg_test/rch_evt_tests/usg_rch_evt.riv

@@ -0,0 +1,3 @@
+         1
+ 1    Stress Period 1
+ 5   3.000000  10.0 2.500000

flopy/modflow/mfriv.py

@@ -238,7 +238,10 @@ def check(self, f=None, verbose=True, level=1, checktype=None):
 
                 # check that river stage and bottom are above model cell
                 # bottoms also checks for nan values
-                botms = self.parent.dis.botm.array[inds]
+                if self.parent.structured:
+                    botms = self.parent.dis.botm.array[inds]
+                else:
+                    botms = self.parent.disu.bot.array[inds]
 
                 for elev in ["stage", "rbot"]:
                     txt = "{} below cell bottom".format(elev)