Separate FFT periodic from FFT OPEN

Summary

We are inconsistent in setting the solvers. For both FFT and FFTOPEN, we are using OPEN solver from IPPL.

https://gitlab.psi.ch/OPAL/opal-x/src/-/blob/master/src/PartBunch/FieldSolver.cpp?ref_type=heads#L316

 else if (this->getStype() == "FFT") {
            if constexpr (Dim == 2 || Dim == 3) {
#ifdef OPALX_FIELD_DEBUG
                this->dumpScalField("rho");
                call_counter_m++;
#endif

                std::get<OpenSolver_t<double, 3>>(this->getSolver()).solve();
#ifdef OPALX_FIELD_DEBUG
                this->dumpScalField("phi");
                this->dumpVectField("ef");
                call_counter_m++;
#endif
            }
        } else if (this->getStype() == "P3M") {
            if constexpr (Dim == 3) {
                std::get<FFTTruncatedGreenSolver_t<double, 3>>(this->getSolver()).solve();
            }
        } else if (this->getStype() == "FFTOPEN") {
            if constexpr (Dim == 3) {
#ifdef OPALX_FIELD_DEBUG
                this->dumpScalField("rho");
                call_counter_m++;
#endif
                std::get<OpenSolver_t<double, 3>>(this->getSolver()).solve();
#ifdef OPALX_FIELD_DEBUG
                this->dumpScalField("phi");
                this->dumpVectField("ef");
                call_counter_m++;
#endif
            }

https://gitlab.psi.ch/OPAL/opal-x/src/-/blob/master/src/PartBunch/FieldSolver.cpp?ref_type=heads#L263

if (this->getStype() == "FFT") {
        initOpenSolver();    
    } else if (this->getStype() == "FFTOPEN") {
        initOpenSolver();    
    }

where

https://gitlab.psi.ch/OPAL/opal-x/src/-/blob/master/src/PartBunch/FieldSolver.cpp?ref_type=heads#L246

template <>
void FieldSolver<double,3>::initOpenSolver() {
        ippl::ParameterList sp;
        sp.add("output_type", OpenSolver_t<double, 3>::SOL_AND_GRAD);
        sp.add("use_heffte_defaults", false);
        sp.add("use_pencils", true);
        sp.add("use_reorder", false);
        sp.add("use_gpu_aware", true);
        sp.add("comm", ippl::p2p_pl);
        sp.add("r2c_direction", 0);
        sp.add("algorithm", OpenSolver_t<double, 3>::HOCKNEY);
        initSolverWithParams<OpenSolver_t<double, 3>>(sp);
}

I think it makes sense to maintain periodic FFT, and correctly separate FFT (periodic) from FFTOPEN solvers. Periodic FFT was working on GPUs, and maintaining it helps with further development while the FFTOPEN being resolved.