Include band center wavelengths in data and/or metadata

[pont-us]

it could also be very useful to have the band center wavelengths stored in the dataset attributes, e.g. as enmap_dataset.attrs["wavelengths"] (or something similar, not sure what the best convention would be).

The metadata file already contains <wavelengthCenterOfBand> for every band. In principle, the wavelengths are always the same, but I think sometimes they can vary slightly depending on calibration, version, etc. It would be nice to have them included in the dataset attributes as a list, so users can be sure they are working with the exact values from the product.

Originally posted by @remoteJorge in #3

Certainly it would be no problem to extract those wavelengths from METADATA.XML and add them as attribute values. Additionally, we could add them as a an alternative co-ordinate set for the band dimension, which would enable things like slice selection/subsetting directly with wavelength ranges. @remoteJorge Do you think that could be useful?

[remoteJorge]

Definitely! That would be great. I always prefer working in wavelengths rather than bands. Band numbers are simpler for indexing, so having both options would be ideal. I just wasn’t sure how much complexity that might add, but as you said, it could be very handy for certain tasks. Another data that would be great to have as attribute (though completely optional) is the FWHM for each band.

[pont-us]

In addition to custom handling of particularly useful metadata like band centre wavelengths, we might consider automatically converting all the metadata (several MB of XML in the product archive) into attributes. Done in the simplest way, this would give all the values as strings, but we could probably apply sensible data type conversions if I can get hold of the XML schema used for EnMAP metadata (so far I couldn't track it down).