[feature] Provide a way in the SPARC2 alignment tab to override the default settings for the focus streams

[pieleric]

We have a "clever" function that can find good settings for the focus stream most of the time. However, due to the large variations in the SPARCs (and especially the cameras), there are a few cases which would work better with different settings.

To handle such SPARCs, allow the service engineer to define a file ~/.config/odemis/focus_settings.json with the name of the camera, and the "good" settings to use. An example of such file content:

[
{
    "name": "VisNIR Camera",
    "detExposureTime": 0.01
}
]