DATAMONGO-941 - Add support for Update $min.

We now offer dedicated methods for _min_ (`Number`/`Date`) on `Update`.
As `BigInteger` and `BigDecimal` values are converted to `String` before storing in `MongoDB` it is not possible to use for the _min_ operation. Therefore we type check both the given value and the property it should be applied to and raise an error if  it fails.

This PR ist a first draft for discussion and should not be directly merged.