Support validator in field() definitions

[smarie]

Spec:

• both as field() argument and as @<field>.validator(...) decorator. Both can be specified in which case the arguments are used first.
• if argument: a possibly ordered dictionary of <error_msg>: <validator>. For old python support, a list of key/val tuples is supported.
• <validator> should be a callable accepting one, two or three arguments (val), (obj, val) or (obj, field, val) so that it has access to the full object on value modification (it can check values of other fields)
• <validator> should return True or None in case of success (same definition than valid8)
• when provided as a decorator, the error message is the argument of the decorator: @<field_name>.validator(<error message>)

[smarie]

For argument mode: It will probably be better to have the dict the other way around (key is validator, value is error message or error type)

[smarie]

We'll use valid8 for this. Version 5.0.0 has been developed to integrate all requirements from here.

Concerning the decorator feature @<field>.validator(...) I opened a separate ticket #9 .

[smarie]

Fixed in 6917b87