semcomedi: increase possible dwell time

Move max dwell time to 100s: unlikely with normal acquisition, but quite possible in CL (with SPARC)
andorcam2: add a second readoutrate to the fake camera

this allows to test the GUI with multiple readoutrates
gui: some fixes on radio buttons for VAs

default to formatted value for the VA

if choices is a dict, correctly display the values as format

Don't override readoutRate VA, as it's now correctly handled by default
gui: fix Slider.Disable()

The code in abstractmethods do count. So Disable() was not working anymore as
it was replaced by a pass.

Also ensure that Enable(False) is the same as Disable(). So now Sliders only have
to implement Enable()