Saving trace output

[brendan-m-murphy]

I noticed that the save_trace option was removed from fixedbasisMCMC. I think it's important to have access to the trace to diagnose problems with inversions.

I will reinstate this after Eric is done merging in the changes for CO2.

What problems was it causing?

[brendan-m-murphy]

So I had a look, and the issue is that the in the function signature, save_trace: Optional[Union[str, Path]] = None was changed to save_trace = False. The way it works is that if you specify a path to save the trace, it will save it, otherwise it won't. The default value None will evaluate to false, so it won't save the trace if you don't specify a path to save it on.