Improve Issie Truth Tables

[tomcl]

The Issie truth table code has been changed by Yujie when he refactored the simulator:

• Check it works - document the UI and anything about the UI which could be improved
• Make it work for extra components not originally included (e.g. NOT gates)
• Check issues here and based on feedback improve the UI.

In addition the Truth table UI is annoying in one obvious respect. Generally when you want algebraic inputs you want all possible ones to be algebraic. Make it so that:

• when no inputs are algebraic, moving to the "algebraic inputs" dialog sets default values which are possible and maximally algebraic
• Add feedback to the dialog so that inputs which cannot be made algebraic are annotated with the reason, and total number of input restriction is obviously visible.