Esmvaltool: fx file retrieval for OBS

[valeriupredoi]

This is not currently implemented. The strategy is as @mattiarighi mentions in #423 :-1:
```For OBS, I would suggest the following fx_file
[project]_[model]_[type]_[version]_fx_[var].nc

We can change it later if we come up with a better idea.```

I will get this done today or tomorrow

[mattiarighi]

If it works for the models, it should work out-of-the-box also for the obs.
We might need some renaming in our obs-data pool.

[valeriupredoi]

aye, @mattiarighi I will close the issue once we've tested and all looks to be a-okay!

[koldunovn]

@valeriupredoi I use it already for my OBS data, it works fine.

[valeriupredoi]

excellent, cheers @koldunovn , closed shall remain then :grin: