Change units of cross section plots to be in MeV

[shimwell]

task 1 has several notebooks with plotting sections where we plot in the default eV units on the X axis

for example

fig = openmc.plotter.plot_xs(
    reactions = {
        'Li6': ['(n,Xt)'],
        'Li7': ['(n,Xt)'],
        'Be9': ['(n,2n)']
    },
)

could be changed to

fig = openmc.plotter.plot_xs(
    reactions = {
        'Li6': ['(n,Xt)'],
        'Li7': ['(n,Xt)'],
        'Be9': ['(n,2n)'],
    },
    energy_axis_units='MeV'
)