Adds _from_memoized_dict to TPState so serialization works again.

After changing signature of __init__ to include 'basis' arg, we forgot
to add this function, which overrides the DenseState implementation and
correctly supplies basis=None to skip 1st-element checking.

Author: Erik Nielsen

pygsti/modelmembers/states/tpstate.py

@@ -14,6 +14,7 @@
 import numpy as _np
 
 from pygsti.baseobjs import Basis as _Basis
+from pygsti.baseobjs import statespace as _statespace
 from pygsti.modelmembers.states.densestate import DenseState as _DenseState
 from pygsti.modelmembers.states.state import State as _State
 from pygsti.baseobjs.protectedarray import ProtectedArray as _ProtectedArray
@@ -53,12 +54,13 @@ class TPState(_DenseState):
     # alpha = 1/sqrt(d) = 1/(len(vec)**0.25).
     def __init__(self, vec, basis="pp", evotype="default", state_space=None):
         vector = _State._to_vector(vec)
-        if not isinstance(basis, _Basis):
-            basis = _Basis.cast(basis, len(vector))  # don't perform this cast if we're given a basis
-        firstEl = basis.elsize**-0.25  # not dim, as the dimension of the vector space may be less
-        if not _np.isclose(vector[0], firstEl):
-            raise ValueError("Cannot create TPState: "
-                             "first element must equal %g!" % firstEl)
+        if basis is not None:
+            if not isinstance(basis, _Basis):
+                basis = _Basis.cast(basis, len(vector))  # don't perform this cast if we're given a basis
+            firstEl = basis.elsize**-0.25  # not dim, as the dimension of the vector space may be less
+            if not _np.isclose(vector[0], firstEl):
+                raise ValueError("Cannot create TPState: first element must equal %g!" % firstEl)
+        # if basis is None, don't check first element (hackfor de-serialization, so we don't need to store basis)
 
         _DenseState.__init__(self, vector, evotype, state_space)
         assert(isinstance(self.columnvec, _ProtectedArray))
@@ -188,3 +190,9 @@ def has_nonzero_hessian(self):
         bool
         """
         return False
+
+    @classmethod
+    def _from_memoized_dict(cls, mm_dict, serial_memo):
+        vec = cls._decodemx(mm_dict['dense_superket_vector'])
+        state_space = _statespace.StateSpace.from_nice_serialization(mm_dict['state_space'])
+        return cls(vec, None, mm_dict['evotype'], state_space)  # use basis=None to skip 1st element check