private <O> Signature<I, O> signature(Class<O> outputClass) {
    return new Signature<>(input, outputClass);
  }
Attempts to evaluate {@code expression} from a yaml config, using only globally-available environmental variables
from the {@code root} context.
planetiler-custommap/src/main/java/com/onthegomap/planetiler/custommap/ConfigExpressionParser.java