Fix some POD errors
Query::SkyCat: fix cfg_file method

This is a class method and so it must not treat its first
argument as an instance of the class!
4_skycat_gsc.t: set SkyCat config first

In case the user has a SkyCat configuration file which does not
include "gsc@eso", the configuration file must be set before
constructing the catalog object.
Use strict in all modules
Prepare v4.31 release