Improve description of PSF axes

[GernotMaier]

Proposal to clarify the meaning of the axes in the PSF histograms (see also issue #174)

[maxnoe]

AE vs BE? Do we have a guideline for that? I would say AE

[maxnoe]

Please keep "axis", as this refers to the axis definition of the matrix

[TarekHC]

I would not say "Array", as this may be used by non-IACT instruments...

[GernotMaier]

But then some one should change https://gamma-astro-data-formats.readthedocs.io/en/latest/general/coordinates.html#coords-fov , as here it is explicitly said that these types of coordinates are used for IACTs and that the centre of the FOV is the centre of the array pointing direction.

[maxnoe]

@TarekHC I would also say that the issue of rewording everything so it also works e.g. for HAWC is a larger one and should probably be done by someone from the Water Cherenkov community.

[maxnoe]

There are too many places where this is currently an issue.

[maxnoe]

Add axis here to be consistent.

[maxnoe]

It's also not mentioned that these columns are lower and upper bin edges respectively. Maybe we should write it like this?

Columns:

* ``ENERG_LO`` -- ndim: 1, unit: TeV
    Lower bin edges of the true energy axis
* ``ENERG_HI`` -- ndim: 1, unit: TeV
    Upper bin edges of the true energy axis 
* ``THETA_LO`` -- ndim: 1, unit: deg
    Lower bin edges of the axis in the angular distance to center of the array pointing direction (see :ref:`coords-fov`)
*  ``THETA_HI`` -- ndim: 1, unit: deg
    Upper bin edges of the axis in the angular distance to center of the array pointing direction (see :ref:`coords-fov`)
* ``RAD_LO``  -- ndim: 1, unit: deg
     Lower bin edges of the axis in angular distance between simulated and reconstructed direction
* ``RAD_HI`` -- ndim: 1, unit: deg
    Upper bin edges of the axis in angular distance between simulated and reconstructed direction
* ``RPSF`` -- ndim: 3, unit: sr^-1
   Matrix of point spread function values in the binning defined in the above columns :math:`dP/d\Omega`, see :ref:`psf-pdf`.