default void setDecimalDigits(Integer digits) {
        put(HasDecimalDigitsUtil.DECIMAL_DIGITS, digits);
    }
Mutates the {@link HasDecimalDigits#getDecimalDigits() decimal digits} of
the underlying document.

@param digits  the new decimal digits count (or {@code null}).
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/mutator/trait/HasDecimalDigitsMutator.java