Compatibility fixes for more modern Perl (5.18)

This PR includes minor Perl tweaks to allow starcheck to work with a more modern Perl.  These changes include (so far).

* Explicitly place parens around qw() lists as needed (required as of Perl 5.14)
* Update time checks in the dark current checker to use a numeric tolerance instead of relying on equality (this is required if we officially deprecate the hack code of date2time and use Chandra::Time everywhere).

With these changes (plus an updated Ska::Convert (date2time) package, an updated Ska.Matplotlib, and an updated xija) the current starcheck regression tests pass acceptably with a 64 bit conda skare.