better support for custom model editing

[pkienzle]

As part of the debug process the model file will frequently be saved and edited. Having to reselect the model and reenter the parameters each time is tedious.

Users may be editing models with their own editor outside of sasview. There is no way to signal that the model needs to be updated.

Both issues could be avoided if the Compute button checks for updates to the model file before running compute, possibly replacing the model with a newer version and displaying any errors with the reload process. Copy the existing values for the parameters to the newly loaded model for all parameters which match.