Static leap-seconds file could be out-of-date

[trey-stafford]

The package itself contains a static copy of tai-utc.dat without, it seems, the code to check if the file is out-of-date for the target GPS time. This package was written with (pre)OIB data in mind, which is not getting updated/added to, so the static file would work every time. As a more generic library (which we want), this approach could fail in the future when leap seconds are added/removed.

Related: #2