[Bug fix] Restarts messed up u/a

Both u and a were written to and read from the field 'u'

src/hdf5/restart-io.cc

@@ -21,7 +21,7 @@ template <class ProgramState>
 void RestartIO<ProgramState>::write(ProgramState const &programState) {
   addEntry(displacementWriter_, programState.timeStep, programState.u);
   addEntry(velocityWriter_, programState.timeStep, programState.v);
-  addEntry(accelerationWriter_, programState.timeStep, programState.u);
+  addEntry(accelerationWriter_, programState.timeStep, programState.a);
   addEntry(stateWriter_, programState.timeStep, programState.alpha);
   addEntry(weightedNormalStressWriter_, programState.timeStep,
            programState.weightedNormalStress);
@@ -37,7 +37,7 @@ void RestartIO<ProgramState>::read(size_t timeStep,
   programState.timeStep = timeStep;
   readEntry(displacementWriter_, timeStep, programState.u);
   readEntry(velocityWriter_, timeStep, programState.v);
-  readEntry(accelerationWriter_, timeStep, programState.u);
+  readEntry(accelerationWriter_, timeStep, programState.a);
   readEntry(stateWriter_, timeStep, programState.alpha);
   readEntry(weightedNormalStressWriter_, timeStep,
             programState.weightedNormalStress);