mktables: Move 'format' to base class

mktables: Move 'format' to base class

This patch is in preparation for adding more complete annotations to the
output tables.  Previously, only Map tables had a listed format.  Match
tables also have a format (empty), but that was understood.  But the new
annotation code will need to refer to that format, so I moved the field
into the base class common to both types of tables, and added the
EMPTY_FORMAT for use by Match tables.