AcceleratorBuffer: Use exp-val-z extra info for getExpectationValueZ() if present

Unconditonally return exp-val-z from AcceleratorBuffer extra info as
getExpectationValueZ() if exists. If we have a more precise result at
hand, why not use it instead of calculating a less precise answer?
Signed-off-by: Austin Adams <aja@gatech.edu>

xacc/accelerator/AcceleratorBuffer.cpp

@@ -424,9 +424,15 @@ const double AcceleratorBuffer::getExpectationValueZ() {
     return 1;
   };
 
-  if (bitStringToCounts.empty() && this->hasExtraInfoKey("exp-val-z")) {
+  if (this->hasExtraInfoKey("exp-val-z")) {
     aver = mpark::get<double>(getInformation("exp-val-z"));
   } else {
+    if (bitStringToCounts.empty()) {
+      xacc::error("called getExpectationValueZ() on an AcceleratorBuffer with "
+                  "no measurements!");
+      return 0;
+    }
+
     for (auto &kv : bitStringToCounts) {
       int i = std::stoi(kv.first, nullptr, 2);
       auto par = has_even_parity(i);

xacc/accelerator/tests/AcceleratorBufferTester.cpp

@@ -55,6 +55,19 @@ TEST(AcceleratorBufferTester, checkGetExpectationValueZ) {
   EXPECT_TRUE(std::fabs(b.getExpectationValueZ() - 0.178955078125) < 1e-6);
 }
 
+TEST(AcceleratorBufferTester, checkGetExpectationValueZPrioritizesExtraInfo) {
+
+  AcceleratorBuffer buffer("qreg", 5);
+
+  // Add decoy measurements, which have expected value of Z = 1
+  buffer.appendMeasurement("00000", 500);
+  // Add actual expectation we want
+  buffer.addExtraInfo("exp-val-z", -1.0);
+
+  // This should draw from extra info, not calculate from measurements
+  EXPECT_EQ(buffer.getExpectationValueZ(), -1.0);
+}
+
 TEST(AcceleratorBufferTester, checkLoad) {
   const std::string bufferStr = R"bufferStr({
     "AcceleratorBuffer": {