clang compiler error: unmatched template parameter `const CenteringEnum*`

Summary

Clang compiler errors for ippl unit test BCond.

Steps to reproduce

-- The C compiler identification is Clang 9.0.1
-- The CXX compiler identification is Clang 9.0.1
-- Check for working C compiler: /opt/local/bin/mpicc-mpich-clang90

Relevant logs and/or screenshots

[ 97%] Building CXX object tests/CMakeFiles/opal_unit_tests.dir/ippl_src/Field/BCond.cpp.o
In file included from /Users/jsnuverink/Documents/OPAL/fork/src/tests/ippl_src/Field/BCond.cpp:7:
In file included from /Users/jsnuverink/Documents/OPAL/fork/src/ippl/src/FieldLayout/CenteredFieldLayout.h:127:
/Users/jsnuverink/Documents/OPAL/fork/src/ippl/src/FieldLayout/CenteredFieldLayout.hpp:313:3: error: no matching
      function for call to 'centeredInitialize'
  centeredInitialize(*this, mesh, p, vnodes);
  ^~~~~~~~~~~~~~~~~~
/Users/jsnuverink/Documents/OPAL/fork/src/tests/ippl_src/Field/BCond.cpp:411:38: note: in instantiation of member
      function 'CenteredFieldLayout<2, UniformCartesian<2, double>, CartesianCentering<&CCCEnums<2, 2,
      0>::vectorFace, 2, 2> >::CenteredFieldLayout' requested here
    CenteredFieldLayout<Dim,M,vFace> layoutVFace(mesh);
                                     ^
/Users/jsnuverink/Documents/OPAL/fork/src/ippl/src/FieldLayout/CenteredFieldLayout.hpp:93:1: note: candidate
      template ignored: substitution failure : deduced non-type template argument does not have the same type as
      the corresponding template parameter ('CenteringEnum *' vs 'const CenteringEnum *')
centeredInitialize(CenteredFieldLayout<Dim,Mesh,
^
...

Possible fixes

The template parameter CenteringEnum* always has a const qualifier, for example, https://gitlab.psi.ch/OPAL/src/blob/master/ippl/src/Meshes/CartesianCentering.h#L35:

template<const CenteringEnum* CE, unsigned Dim, unsigned NComponents=1U>
class CartesianCentering
{

However the const qualifier seems too restrictive and I propose to remove it.