Change the SIMD command correctly with precision = double vs. single and

connect the "Real" default precisoin to a configure flag.
Have RealF, RealD and Real types, where Real is compile target dependent single/double,
RealF is single and RealD is double etc..

tests/Test_multishift_sqrt.cc

@@ -56,7 +56,7 @@ int main (int argc, char ** argv)
   Grid_init(&argc,&argv);
 
   GridCartesian *grid = SpaceTimeGrid::makeFourDimGrid(GridDefaultLatt(), 
-						       GridDefaultSimd(Nd,vComplexF::Nsimd()),
+						       GridDefaultSimd(Nd,vComplex::Nsimd()),
 						       GridDefaultMpi());
 
   double     lo=0.001;