gf: fix auto_assign for chained calls.

- g[k_][om_] << expression was not working.
 now  implemented like for std::vector adapter.
[API Change] parameters: required, optional

- now chain the required

- pdef.optional( "B", short(0), " short ")
  .optional( "C", 1u, " unsigned short ")
  .optional( "D", int(2), " int ")
  .optional( "E", 3u, " unsigned int ")
  .optional( "F", long(4), " long ")