Sensor list saving in seperate util function

[aviadlevis]

https://github.com/CloudTomography/pyshdom/blob/6215c180ad2e55ed3193511cdf02b53c73e6d398/pyshdom/util.py#L615

Enhancement:
Currently when working on real data it would be useful to have a function just to save the sensor list without needing to create the solver list.

Bug:
I don't follow the logic here - there is an if/else condition with the identical consequences.

[JRLoveridge]

Re: Bug
Ah, previously I was opening the file when saving the first sensor so the 'write' ('w') flag was used in the call .to_netcdf() rather than the append flag ('a'). Since I'm no longer doing that the if/else condition can be removed.

Re: Enhancement
We could just make sensors / solvers keyword arguments defaulted to None and just not use them if they are None?

[aviadlevis]

I think its cleaner to have two separate methods: save_sensors, save_solvers.
The save_forward_model will call both sequentially but you could call each separately.