Fix inconsistent model refresh threading

[johvincau]

When calling openmc-plotter with the -s flag and specifying threads, this number would only be remembered for the first time loading the model. Any subsequent reloads called from the GUI would function as if openmc-plotter were run with no thread specification, returning to the default value for that machine.

This pull request adds a quick fix to enable any future loads/reloads of the model to use the number of threads as specified by the initial call of openmc-plotter.

[paulromano]

Thanks for fixing this! Also looks like the reload would have missed out on the model_path argument and I see you've included that too 👍