Clang compiler warning for IntTSC (-Wsign-compare)

Summary

Clang compiler warning for IntTSC (-Wsign-compare).

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

ippl/src/Particle/IntTSC.h:284:8: error: comparison of integers of
      different signs: 'unsigned int' and 'int' [-Werror,-Wsign-compare]
                if (p==-1) return .125*(1.-4.*dpos(i)+4.*dpos(i)*dpos(i));
                    ~^ ~~

and many similar ones

Possible fixes

The warning comes from https://gitlab.psi.ch/OPAL/src/blob/master/ippl/src/Particle/IntTSC.h#L283

	auto W = [dpos](unsigned p, unsigned i) {
		if (p==-1) return .125*(1.-4.*dpos(i)+4.*dpos(i)*dpos(i));
		else if (p==0) return .25*(3.-4.*dpos(i)*dpos(i));
		else if (p==+1) return .125*(1.+4.*dpos(i)+4.*dpos(i)*dpos(i)); };

p should be a signed integer instead of unsiged.