Pyspex

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* spx1_ccsds2l1a.py: you can now combine multiple files with CCSDS packages from the same measurement into one Level-1A product
* CCSDS: improved verbosity and ignore incomplete images

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* Method DEMio::read_hdr can now return the raw detector header register data, or return this info written into a MPS record
* Fixed: replaced depreciated pyspex.lib.sw_version with pyspex.version

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* Fixed several bugs introduced with the new LV1io classes
* Removed obsolete modules l1a_h5.py and mps_def.py
* Renamed lib.sw_version.py to version.py
* Rearranged source tree to comply with PIP 517, 518 (requires: setuptools 42 or later)
* More work on package setup, many bugs and typo's fixed
* INSTALL.md: updated installation instructions
* Updated Copyright line in all modules
* Updated LICENSE file
* Added ChangeLog.md