Convert segment_lengths when transforming lat and lon coordinates

[jmilloy]

Description
The segment_lengths of lat and lon coordinates need to be transformed when the coordinates are transformed. Currently, a warning is issued. The segment_lengths for alt coordinates are converted using the alt_units and/or proj4 vunits, when applicable, and we may be able to do the same for lat and lon using the proj4 +units, though I'm not sure if this will always be possible.

This may be an intractable issue. It is not clear to me if the segments will even make sense when transformed between some reference systems.

Notes:

• this does not apply when ctype='point'.
• it may be useful to consider this issue along with Interpolation ignores ctype #238
• RotatedCoordinates has a similar and possibly intractable issue with segment_lengths (especially when converted to DependentCoordinates); segment lengths are not handled correctly there as well.

[jmilloy]

I don't think it is possible to do this in most cases. I am still strongly in favor of removing segment_lengths from Coordinates (#266), which would "solve" this issue.

[jmilloy]

We are going to remove segments from Coordinates, so the transform will not need to handle them. See #357.