Add new UNKNOWN type to ElectronicType

[sphuber]

The UNKNOWN type should indicate that the user doesn't know the electronic type of the material and the input generator should use inputs for a best guess. Typically this would be probably to treat the system as a metal with some smearing. The already existing type AUTOMATIC was being used for this purpose, but we want to reserve this option for the use case where the generator actually generates a builder for a workflow that will try and determine the electronic type through actual simulation. This may be too ambitious and it is not sure if this will be implemented in the future.

See for other discussions #211 and #176