Disambiguate THETA Keywords

[maxnoe]

Most (if not all) IACTs call the distance from the reconstructed gamma-ray position to the suspected source position theta, in wobble observation mode, background regions are used and this results in theta_off_* attributes.

The current version of the standard calls the distance of the reconstructed source position to the camera center THETA.

I think we should disambiguate this by adding subscripts like _FOV, _OBJ, _SRC, _ON, _OFF_*

[cdeil]

In Gammapy we have this: https://github.com/gammapy/gammapy/pull/1295/files

For the spec, given that these formats are in use and implemented in several IRF exporters and science tools, there is a cost to changing axis names for consistency.

I'm putting this on v0.3 milestone, not for the v0.2 which we should try to finish up in the next days.