Pharo Glorp platform decimal to ScaledDecimal mapping

Hi Everyone,

I've added mapping from platform decimal to ScaledDecimal for Pharo / Glorp.

It's currently on the development branch and I'll move it to the
master branch in a few days, assuming I don't hear of any issues.

Pharo Glorp repository: https://github.com/pharo-rdbms/glorp

Cheers,
Alistair