fix Snakefile names

tools/graphlan/export2graphlan/Snakefile

tools/graphlan/graphlan/Snakefile

tools/graphlan/graphlan_annotate/Snakefile

tools/metaphlan2/metaphlan2/Snakefile

tools/metaphlan2/metaphlan2/usage_example.rules

tools/metaphlan2/metaphlan2_heatmap/Snakefile

tools/metaphlan2/metaphlan2_heatmap/usage_example.rules

tools/metaphlan2/metaphlan2_merge/Snakefile

tools/metaphlan2/metaphlan2_merge/usage_example.rules

workflows/simple_metaphlan2/Snakefile

workflows/subworkflows/README.md

workflows/subworkflows/graphlan_from_metaphlan2/Snakefile