Add in missing return_basis implementation

Patch in support for the missing support for the return_basis of EmbeddedErrorgen and EmbeddedOp.

Author: Corey Ostrove

pygsti/modelmembers/operations/embeddederrorgen.py

@@ -139,16 +139,26 @@ def coefficients(self, return_basis=False, logscale_nonham=False, label_type='gl
             Where `termType` is `"H"` (Hamiltonian), `"S"` (Stochastic),
             `"C"`(Correlation)  or `"A"` (Affine).  Hamiltonian and S terms always have a
             single basis label while 'C' and 'A' terms have two.
-        """
-        coeffs_to_embed = self.embedded_op.coefficients(return_basis, logscale_nonham, label_type)
 
+        basis : `Basis` (if return_basis==True)
+            A Basis mapping the basis labels used in the keys of basis-labels of the
+            underlying (pre-embedding) error generator coeffs to basis matrices.
+        """
+        if return_basis:
+            coeffs_to_embed, basis = self.embedded_op.coefficients(return_basis, logscale_nonham, label_type)
+        else:
+            coeffs_to_embed = self.embedded_op.coefficients(return_basis, logscale_nonham, label_type)
+        
         if coeffs_to_embed:
             embedded_labels = self.coefficient_labels(label_type=label_type, identity_label=identity_label)
             embedded_coeffs = {lbl:val for lbl, val in zip(embedded_labels, coeffs_to_embed.values())}
         else:
             embedded_coeffs = dict()
 
-        return embedded_coeffs
+        if return_basis:
+            return embedded_coeffs, basis
+        else:
+            return embedded_coeffs
 
     def coefficient_labels(self, label_type='global', identity_label='I'):
         """

pygsti/modelmembers/operations/embeddedop.py

@@ -600,17 +600,27 @@ def errorgen_coefficients(self, return_basis=False, logscale_nonham=False, label
             Where `termType` is `"H"` (Hamiltonian), `"S"` (Stochastic),
             `"C"`(Correlation)  or `"A"` (Affine).  Hamiltonian and S terms always have a
             single basis label while 'C' and 'A' terms have two. 
+        
+        basis : `Basis` (if return_basis==True)
+            A Basis mapping the basis labels used in the keys of basis-labels of the
+            underlying (pre-embedding) error generator coeffs to basis matrices.
         """
         #*** Note: this function is nearly identical to EmbeddedErrorgen.coefficients() ***
-        coeffs_to_embed = self.embedded_op.errorgen_coefficients(return_basis, logscale_nonham, label_type)
-        
+        if return_basis:
+            coeffs_to_embed, basis = self.embedded_op.errorgen_coefficients(return_basis, logscale_nonham, label_type)
+        else:
+            coeffs_to_embed, basis = self.embedded_op.errorgen_coefficients(return_basis, logscale_nonham, label_type)
+
         if coeffs_to_embed:
             embedded_labels = self.errorgen_coefficient_labels(label_type=label_type, identity_label=identity_label)
             embedded_coeffs = {lbl:val for lbl, val in zip(embedded_labels, coeffs_to_embed.values())}
         else:
             embedded_coeffs = dict()
 
-        return embedded_coeffs
+        if return_basis:
+            return embedded_coeffs, basis
+        else:
+            return embedded_coeffs
 
     def errorgen_coefficient_labels(self, label_type='global', identity_label='I'):
         """