Some thermal device models excluded from gen stack in plot_fuel()

[ajsauter]

For the v015 update, there is at least one thermal device model that does not translate well to plot_fuel().

ThermalCompactUnitCommitment only finds the solution for the power above minimum, meaning the "ActivePowerVariable__ThermalMultistart" variable is not generated. It looks like there is an AuxVariable generated in place of this, but plot_fuel() is not yet searching for that AuxVariable when "ActivePowerVariable__ThermalMultistart" is absent from sim results. Hopefully this is a trivial fix for the next update to PowerGraphics.