fixed incorrectly detected error by mypy on some systems
black has reformatted code after update

added support for strings
this is a bit more tricky so issues could arise

also fixed ess.__name__
see https://packaging.python.org/guides/single-sourcing-package-version/

added more options to the data file

this is the first list of options added

includes support for converting 3D quad fields
found a minor error in field_on_axis, slightly wrong mid index was selected
some minor cleaning included

using loadtxt rather than genfromtxt
(did not need genfromtxt)
using skiprows and comments instead of header
do not assume z coordinate start at 0
Used to generate MEBT field maps

updated the test file for density v11

I would normally expect for density file v11,
but I have earlier corrected it.
Now it was not working with our example file so I
corrected back.

- more consistent module names
- removing a duplicate
- fixing new flake8 warnings

fix minor issue in DTL_CEL class