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

some minor cleaning as well

Supporting python 3.8 as well
adding missing field maps for tests
version checker added
version checker needs all deps installed

updated style
fix in lib_tw, was catching the wrong type of error
adding some tests for lib_tw

correcting the amount of data per step in v11 Density_PAR?

Thanks to Ibon for finding this

we do not support python 2 anymore