move zdc pi0 and photon benchmarks from physics benchmarks (#70)

.gitlab-ci.yml

Snakefile

benchmarks/zdc_photon/Snakefile

benchmarks/zdc_photon/analysis/gen_particles.cxx

benchmarks/zdc_photon/analysis/zdc_photon_plots.py

benchmarks/zdc_photon/config.yml

benchmarks/zdc_pi0/Snakefile

benchmarks/zdc_pi0/analysis/gen_pi0_decay.cxx

benchmarks/zdc_pi0/analysis/zdc_pi0_plots.py

benchmarks/zdc_pi0/config.yml