CDAWEB SPDF files loading

[Beforerr]

Some files are available in SPDF, but could not be loaded to speasy: for example, L1 files for THEMIS https://spdf.gsfc.nasa.gov/pub/data/themis/tha/l1/ .

It would be nice we could support loading them.

[jeandet]

Sure, depending on how much they are ISTP compliant, it will either require a simple fix inside Speasy or a dedicated codec.
Depending on the complexity, we can target the next release.