Added fname_base feature

[John-Ragland]

I've played around with adding a feature where you can pass a custom file name to the model, and it will use this instead of the temp files. The default is None, which uses the temp files as normal.

Example usage (works with compute_arrivals, compute_transmission_loss, compute_rays)

arrivals = pm.compute_eigenrays(env, fname_base='test')

printed output: [CUSTOM FILES] Bellhop working files: arlpy_test.*

Let me know what you think!

[mchitre]

In concept, no objections to it, but it would be good to understand the use case before commenting. Could you elaborate where you see this being used?

[John-Ragland]

it's a fairly niche use case, but would allow having more control over the where the env and prt files are stored.