fix(mp7): fix shape assignment for DISU package (#2651)

Author: maseology

flopy/modpath/mp7.py

@@ -144,7 +144,7 @@ def __init__(
                     shape = (nlay, ncpl)
                 elif dis.package_name.lower() == "disu":
                     nodes = dis.nodes.array
-                    shape = tuple(nodes)
+                    shape = (nodes,)
                 else:
                     raise TypeError(
                         "DIS, DISV, or DISU packages must be "