Add no_unit to duration_style.

[rkawulak]

When using Chrono I/O V2, currently there's no easy way to specify that only the numeric value of a duration without the unit should be output to a stream (equivalent of duration unit pattern "%v"). It would be useful to add a new enumerator, say 'no_unit', to duration_style enumeration, and a corresponding stream manipulator (e.g. 'no_unit_format').

[viboes]

Yes, this can be useful. A patch would be welcome.

[rkawulak]

It seems that a more flexible approach would be to allow changing the pattern of duration_units facet (currently that's not possible since there's no interface to modify the pattern). Unfortunately I won't be able to provide a patch at the moment.