Fix map write for custom axis name #1433
The key change is to use
The change in the test to add an axis
With this implementation, when the axis name is energy, the behavior is then to have column names:
This seems good to me as it is likely transparent for the user anyway. This might be what is actually stated in [the definition of the BANDS HDU] (http://gamma-astro-data-formats.readthedocs.io/en/latest/skymaps/index.html#bands-hdu).