user api interface

[shimwell]

this could be turned into a package.

perhaps user interface would look like this

xs, energy = get_xs(isotope('Li6', library='ENDF7.1',reaction='n,t')
)

perhaps EXFOR exp data could be added

xs, energy = get_xs(isotope('Li7', library='EXFOR', reaction='n,t'))

if we make unified cross sections then we can plot materials

xs, energy = get_xs(
    material(
            [
                (isotope('Li6', library='ENDF7.1'), 0.5),
                (isotope('Li7', library='ENDF7.1'), 0.5),
            ],
            density=1,
            density_units='g/cm3'
            reaction='n,t'
        )
)

perhaps group wise data could be plotted

xs, energy = get_xs(
    isotope('Li6', library='ENDF7.1',reaction='n,t', group_strucutre=['ccfe-709'])
)