PS_TIME_TAI in mdc files

[Paul Price]

Dave Robbins has introduced a PS_TIME_TAI into the metadata config files for the
purpose of representing a psTime. I've no idea if we can read these with psLib,
but they can be written.

• Is this acceptable? Or do we require a different format (e.g., ISO time)?
• Whatever the alternative, the PS::IPP::Metadata::Config->parse must be able to

parse it.

[jhoblitt]

I've updated psLib so that times (UTC, UT1, TAI, TT) are now read in and emitted
in ISO8601 format.