Clarify what kinds of arrays are allowed for IRFs

[maxnoe]

Currently the standard asks for the storage of two arrays of lower and upper bin edges.

In python:

# 50 bins between 1 GeV and 1 PeV
bin_edges = np.logspace(-3, 3, 51)
energ_lo = bin_edges[:-1]
energ_hi = bin_edges[1:]

same goes for theta

I would propose to change this for the next iteration of the standard to change this to just storing a single array with bin edges.