compiler errors in ippl/test with clang9

Summary

Reported by @adelmann.

Compiler errors with clang with ENABLE_IPPLTESTS=ON

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

clang: error: optimization flag '-fno-tree-vrp'

ippl/test/particle/p3m3d.cpp:170:72: error: implicit instantiation of
      undefined template 'ApplyField<double>'
        HPB.for_each(RadiusCondition<double, Dim>(interaction_radius), ApplyField<double>(-1,interaction_radius));

ippl/test/particle/ChargedParticleFactory.hpp:564:73: error: unused
      parameter 'extend_l' [-Werror,-Wunused-parameter]
void createParticleDistributionEquiPart(Particles & P, Vektor<double,3> extend_l, Vektor<double,3> extend_r...

Possible fixes

• remove the -fno-tree-vrp flag
• comment out function parameters
• reorder code to avoid undefined template error