correcting the amount of data per step in v11 Density_PAR?

Thanks to Ibon for finding this

we do not support python 2 anymore

ess.lib_tw is broken due to removing star imports

bump version to 2.3

using meshgrid for the data
added interpolation function

(unsure how this worked before)

new function plt.get_dst returns a dst object
function plt.save_dst forces you to provide a file name (and use get_dst)

- current convert A vs mA
- forgot to write mass (= mc2)
- using file write() rather than print function
print was not working correctly on both python2 and 3
this has been tested to give correct result on both

(was not working on my python2)
added plt.save_dst()
fixed pack function in dst.save() for python3

replaced xrange -> range (py3 compatibility)

made sure forward compatibility is preserved

fix for IS energy levels (use keV instead of MeV when max(E)<0.1)

resembles TraceWin distribution plot

this might not work in all cases just yet

by default false. if true overwrite existing group

also some cosmetic cleaning