Decide what mean the "Mean" button should display

[patrikhuber]

Shall we display the identity mean, or the identity + expression mean (for models with PCA expression models)? Probably we want means to do both. Also depends on what we decide here: https://github.com/patrikhuber/eos/issues/188

[patrikhuber]

0beb6a838b7c0946c5de37408e8c68db5c3efce3 adds two buttons, one to display each.

[patrikhuber]

Re-opening, this got broken by #5. The slider code now always redraws the model, including the expression mean, if a model has expressions. I think a checkbox "Identity only" might be a good idea, or "Use expression model".