TCTracks: Option to read additional variables from IBTrACS (e.g. "nature"/stormtype)

[tovogt]

Changes proposed in this PR:

• Add a parameter additional_variables to the TCTracks.from_ibtracs_netcdf function that allows users to have additional IBTrACS data fields be included in the TCTracks object.

This PR fixes #674 and replaces #646

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[ChrisFairless]

This is great! Looks good, reads well, and everything works.

Only one remark: the additional variables include a few numeric fields (iso_time, wmo_wind, wmo_pres, dist2land, storm_speed, storm_dir). None of these are interpolated when interpolate_missing = True. Do we want to extend the code to interpolate missing values here or leave that to the user?

[tovogt]

Only one remark: the additional variables include a few numeric fields (iso_time, wmo_wind, wmo_pres, dist2land, storm_speed, storm_dir). None of these are interpolated when interpolate_missing = True. Do we want to extend the code to interpolate missing values here or leave that to the user?

These variables are not reported variables, but are provided by IBTrACS as a courtesy to users, e.g. dist2land, storm_speed, storm_dir. They never have missing values as far as I understand it. For wmo_wind and wmo_pres, I think that nobody should use these. If somebody is interested in those values, they should use the provider="official" feature.

The documentation of interpolate_missing includes a complete list of variables that are affected by this setting, and I think that's clear enough.

[ChrisFairless]

Thanks for clarifying! Then I think this is ready to merge.

[bguillod]

This looks good to me.
A big THANK YOU @tovogt for writing code and creating this PR, this is highly appreciated.

[peanutfun]

Great work, @tovogt! Since @chahank is a bit busy at the moment I took the liberty of reviewing your changes.

[tovogt]

Thanks for your reviews, @ChrisFairless, @bguillod, @peanutfun !

From my side, this is ready to be merged.

[peanutfun]

Great, thanks a lot. I'll merge then!