interrupt long C model evaluations

[pkienzle]

Processing time for Compute with large amounts of polydispersity can be long. The polydispersity loop is being executed in small hunks within python, so it would be possible to check an abort flag within the polydispersity calculation. The GUI would need to set the flag and sasmodels would need to have it available. The model may be executing in the GUI thread, in a separate calc thread, in a separate process, or on a separate machine during a fit, so communication needs to be flexible.