Improve get/set camera parameters

[GuLinux]

the methods GetQHYCCDParam and GetQHYCCDParamMinMaxStep are a bit limited at the moment.
For some parameters (exposure, for instance), there should be some kind of context, to be able to transform the value in milliseconds for GUI display. Maybe this particular issue could be solved by just documenting a bit more exposure setting.

On other parameters (TRANSFERBIT) there is no min/max/step reporting: in this case maybe there should be a list of allowed values (8, 12, 16). If you want to keep a standard C interface you could add two optional out parameters on GetQHYCCDParamMinMaxStep: int _fixedValuesSize (number of fixed values, 0 if it's not a fixed value parameter), and double *_fixedValues (pointer to an array of double, 0 if it's not a fixed value parameter). Another parameter, "Type", could help describing how this setting should be displayed in GUI (integer, double, fixed-values)

Same thing could apply for resolution: there isn't currently a "resolution" control, but it could be useful to know camera supported resolutions, and maybe this new parameters interface could be used to display them.

Last, on my camera some controls, particularly gain, seem to be partially broken: I have min=0, max=4, step=1, but if I try changing gain with values lower than step (for instance, 0.2) it still works, so maybe in this case step should be much lower.