adding lethargy option to axis

[shimwell]

It would be nice to allow per unit lethargy axis options to the plots

this would need to get the energy bin width as part of the calculation

[shimwell]

this code might help, thanks to Patrick for this example

# makes the spectrum tally
energy_filter = openmc.EnergyFilter.from_group_structure('CASMO-70')
spectrum_tally = openmc.Tally(name="spectrum tally")
spectrum_tally.filters = [energy_filter]
spectrum_tally.scores = ['flux']

# gets the tally result
with openmc.StatePoint(sp_filename) as statepoint:
    spectrum_tally_out = statepoint.get_tally(id=spectrum_tally.id)

# gets the bin 
bin_boundaries = np.unique(energy_filter.bins).copy()
bin_boundaries[0] = 1e-10

flux = spectrum_tally_out.mean.flatten() # particle / source

# need to normalize flux by the log-width of the energy bins
log_de = np.log10(bin_boundaries[1:]/bin_boundaries[:-1])
flux = flux / log_de

plt.step(np.unique(energy_filter.bins)[:-1], flux)
plt.xscale('log')
plt.show()