CLI argument to specify custom output simulation filename suffix

[btobers]

Closes #80.

outputfn_sfix argument added to run_simulation.py argparser so that a custom output simulation filename suffix can be passed, which may be useful in some cases. For instance, if particular parameters were being updated for a given glacier and then simulations were run, the default output simulation filename would be used and overwrite previous runs.

[btobers]

@ddundo If you have any interest in improving the way simulations are saved, perhaps this would be a good time. I wouldn't be able to work on this for a while, but wanted to mention it to you in case you have the desire to make any improvements on this active PR. I'm sure improvements could be made, but I suspect it'd take quite a bit of work. So unless you have the time to work on doing so, I suggest we close this and further improvements can be made in the future.

One thing I'm not sure about, related to #70, should anything else be included in __all__ for pygem.output?

[ddundo]

Thanks @btobers. I indeed don't have much time now to dig into this, so I'm happy to merge if you are happy.

There are a few things I noticed that are strange though. For example, I think the use of @dataclass here is not appropriate. But yes, we can get back to this later.

As for __all__, it should include any class or function that we want users to use in their scripts.

[btobers]

Sounds good, thanks @ddundo. I'll close this PR and improvements can be made in the future.