Add a new atan()-curve based accel method

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
userstudy: add support for the new accel method

Pick two out of the three to leave the rest of the study in place.
And write out the method name too, easier for human debugging later.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
userstudy-analysis: handle methods with differnt index than 0 or 1

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>