Add bandgap checks so that we use the right type of smearing.

[bocklund]

Right now we always using smearing parameters for metals, but we want to be able to easily do semiconductors or insulators too.

There's a question of how to handle this once we have detected that we are dealing with a material with/without a bandgap (after the relaxation)

1. Change the smearing on they fly for all the future fireworks, redo the relaxation with the correct smearing parameters
2. Fizzle and raise an error that a user would check later and fix manually
3. Defuse this workflow and make an entirely new workflow

Perhaps there should also be a "force" option, in the case that the user knows better for some reason.