Improve PointingInfo class

[cdeil]

In #545 I added a PointingInfo class.

It is unfinished, the following additions should be made in a follow-up pull requests (not all at once ... incremental improvements are needed here):

• Link to spec (when available)
• Correct test with HESS (when differences understood)
• Add interpolation method to get RA / DEC and ALT / AZ at arbitrary times.
• Refactor to avoid duplicated code with the EventList and ObservationTable classes?
• Integrate with DataStore
• Use in high-level IACT analysis code

[cdeil]

Closing this old issue. Half of the tasks above are done. The class is currently in pretty good shape, and anyways it's still unused. If we start using it in the future (e.g. for slowing observations, or for CTA), we can improve it further, but this old issue won't help.