Support global checker #8

if the class contains a __validate_all_fields__(self) method, it will be called everytime a field is modified. This is quite heavy but might be useful in some rare cases where validation can not be done in each of the fields validators

• you can disable this temporarily by doing "with skip_global_validation(o): ..." (implementation note: this would set a __skip_global_validation__ flag on the instance, read by the fields setters)

• or permanently with set_global_validation(o, enabled=False) ?