Add GTI export in datasets

[registerrier]

GTIs have been added to datasets but were not exported in the SpectrumDatasetOnOff.to_ogip() and MapDataset.write() methods. They were also not read in SpectrumDatasetOnOff.from_ogip() and MapDataset.read() methods.

This PR solves this issue by introducing GTI.to_hdulist() and GTI.from_hdulist() methods.

[registerrier]

Thanks. I introduced the to_hdulist and from_hdulist for what I saw as API consistency. It is a bit weird to create an HDUList for a single table indeed and the .table API should be sufficient.
Yet the object being not a Table but rather containing one I found that a specific API was better.

We will see what to do with the API consistency globally as proposed in #2442 .

Merging now.