Fix for particles w/o multipole axis (e.g. ions)

-This line, and th -se below, will be ignored--

M    AmoebaMultipoleForceImpl.cpp

plugins/amoeba/openmmapi/src/AmoebaMultipoleForceImpl.cpp

@@ -66,13 +66,14 @@ void AmoebaMultipoleForceImpl::initialize(ContextImpl& context) {
 
         if( axisType != AmoebaMultipoleForce::ZThenX     && axisType != AmoebaMultipoleForce::Bisector &&
             axisType != AmoebaMultipoleForce::ZBisect    && axisType != AmoebaMultipoleForce::ThreeFold &&
-            axisType != AmoebaMultipoleForce::ZOnly ) {
+            axisType != AmoebaMultipoleForce::ZOnly      && axisType != AmoebaMultipoleForce::NoAxisType ) {
              std::stringstream buffer;
              buffer << "AmoebaMultipoleForce: axis type=" << axisType;
              buffer << " not currently handled - only axisTypes[ ";
              buffer << AmoebaMultipoleForce::ZThenX   << ", " << AmoebaMultipoleForce::Bisector  << ", ";
-             buffer << AmoebaMultipoleForce::ZBisect  << ", " << AmoebaMultipoleForce::ThreeFold;
-             buffer << "] (ZThenX, Bisector, Z-Bisect, ThreeFold) currently handled .";
+             buffer << AmoebaMultipoleForce::ZBisect  << ", " << AmoebaMultipoleForce::ThreeFold << ", ";
+             buffer << AmoebaMultipoleForce::NoAxisType;
+             buffer << "] (ZThenX, Bisector, Z-Bisect, ThreeFold, NoAxisType) currently handled .";
              throw OpenMMException(buffer.str());
         }
     }