Add option to specify number of threads on model training

[alastair]

Tools like make and waf have a -j (jobs) flag to specify the number of threads to use.
In Gaia we just use the number of available threads.

It'd be nice to add this option to train_model_from_sigs and train_model.
Should this option be in the profile file? It sounds to me like it should instead be only a parameter, but train_model calls run_tests.runTests directly, which creates a ClassificationTaskManager (which only takes a profile file as an argument).
In order to get this parameter into ClassificationTaskManager, we might have to pass it as an argument through all of these files.

Ideas?

[dbogdanov]

If we think that profile files are supposed to be shareable in order to reproduce an experiment across machines, then I don't think that the number of threads should really belong there.

[alastair]

so we should definitely take a -j flag and pass it through all of the functions. We can still default to the number of detected cores