Introduce Jam_Field_Decimal

The decimal field is like the float field. But it casts the value to `float` in the `convert` function. So if you perform a validation on float fields from user input you'd want to give a proper validation message with the originally inputted value in the input even if it is not a float.

The class description says it best:
- set it like a string
- validate it like a float
- insert it (into the database) like a decimal

It is best used for fields which are represented in the database schema with a `DECIMAL` column.