Nihilist cryptanalysis consider providing keyword length

[Agilus]

We discussed the possibility of providing the keyword length for Nihilist cryptanalysis problems to make them a bit less time-intensive and more likely to be attempted by competitors.

• Do we want to change the Suggest Question dialog to support this?
• Do we want to provide the keyword length to the AutoSolver?
• If we move forward with it, I can update the TestGuidance.html to include this new guidance.

[toebes]

Need to wait for #425