SgField.validate() logger now prints value of arg "force"

When a fields validate function is called the value of force is now logged.
SgField.__fieldclasses__ cleaned up default value

Default value for the global var __fieldclasses__ contained key data that is no
longer represented in the ORM.
Updated SgField.invalidate() doc string and logger print update

The invalidate() function did not contain any arg doc info, added info about
args.  Logger now also prints the value of the arg "force".