install: script for odemis start accepts to override the default model file

this should allow to easily run different model files.
For instance, at AMOLF, we could run either the spectrometer or the AR backend
gui tabs: avoid failing if SPARC has only one detector

check for the presence of the stream before checking if it has the focus.
config: add simulated SPARC with only one detector

for testing the GUI with such a microscope