Do something useful on enter in a parameter field.

Enter in a parameter field now re-generates the model.  Previously, it
reloaded the page.

Note: The "go" button is still a button that calls go(), not a form
submit control.  This is for debugability.  Execution has to make it to
"return false" to prevent submission.  For form submits, if an error
happens anywhere, the form submits and uselessly reloads the page.

So the "go" button works differently (better) than enter in a parameter
field in case of errors.  This is not ideal, but is functional for now.