Add Val class similar to ValLoc

[MrBurmark]

Add Val class similar to ValLoc so we can have a .min method like what the legacy reducers have.

[MrBurmark]

Through discussion we have decided to move toward changes to the API to bring the operator into the lambda argument to improve type safety.

  DATA_TYPE maxinit = big_max;
  DATA_TYPE max(max_init);

  // RefOp<DATA_TYPE, OPERATION>;
  // using RefLocOp<DATA_TYPE, Index_Type, OPERATION> = RefOp<ValLoc<DATA_TYPE, Index_Type>, OPERATION>;

  template < typename DATA_TYPE >
  using MaxRef = RefOp<DATA_TYPE, RAJA::operators::maximum>;

  RAJA::forall<EXEC_POLICY>(seg, 
    RAJA::expt::Reduce(MaxRef(maxinit)),
    RAJA::expt::Reduce<RAJA::operators::maximum>(&max),
    [=] RAJA_HOST_DEVICE(IDX_TYPE idx, MaxRef<DATA_TYPE> mi, DATA_TYPE &m) {
      mi.max(working_array[idx]);
      mi.min(...); // error (SFINAE)
      m  = RAJA_MAX(working_array[idx], m);
  });

[MrBurmark]

We might want to unify the ValLoc and ValueLoc classes.

[MrBurmark]

We might want to SFINAE out the operators based on an operator template arg. For example Val<double, RAJA::min> only has the min operator and += is disabled.