Add marker legend in trajectories map

src/analysis/assets/markers/obb_ec.svg

src/analysis/assets/markers/other.svg

src/analysis/assets/markers/tper_reg.svg

src/analysis/assets/markers/trenitalia_ec.svg

src/analysis/assets/markers/trenitalia_fa.svg

src/analysis/assets/markers/trenitalia_fb.svg

src/analysis/assets/markers/trenitalia_fr.svg

src/analysis/assets/markers/trenitalia_ic.svg

src/analysis/assets/markers/trenitalia_icn.svg

src/analysis/assets/markers/trenitalia_reg.svg

src/analysis/assets/markers/trenord_reg.svg

src/analysis/assets/templates/marker_legend.html

src/analysis/assets/templates/train_count.html

src/analysis/trajectories_map.py