Fix incorrect unit label when plotting

[lucydot]

When we run e.g.

reaction.Dmu(xc='scan').plot_TvsP(scale_range=[-2,-1])

the energy scale legend shows kJ/mol but the energy is actually in eV by default.

Specifying units gives correct result:

reaction.Dmu(units="kJ",xc='scan').plot_TvsP(scale_range=[-2,-1])

A Must Fix. Ideally the plot would detect what units the energy is in and then label appropriately.