New HexNumeric field

[madhr]

It would be useful to introduce a new HexNumeric field that would allow to store binary encoded fields as hex numeric fields.

It's related to this change:
#227
where it's now possible to treat binary encoded fields as Hex.

Since Hex is a string field, it's a bit inconvenient for fields that store numeric values like amounts (for instance DE55.9F02).
Introducing the HexNumeric allows to convert the binary encoded fields straight into a hex numeric field that allows further arithmetic operations without extra conversion.

I've submitted a pull request here: #301