Calculated sun-earth distance correction factor once and add as attribute

[ninahakansson]

The sun-earth distance correction factor is calculated both in gac_reader and gac_io at the moment. And when using pygac as reader from satpy, there is no access to this attribute.

The calculation should be removed from gac_io. And the value used should be accessible from satpy. If possible in a general way so that other items can be added later without the need to update satpy.

[sfinkens]

That factor is now available via Reader.meta_data which is used by Satpy.