[xgboost] Some formatting.

lib/xgboost/bin/dune

lib/xgboost/bin/parse_xgboost.ml

lib/xgboost/dune

lib/xgboost/example/dune

lib/xgboost/input.ml

lib/xgboost/input.mli

src/convert_xgboost.ml

src/convert_xgboost.mli

src/main.ml