Implement the compression entity header as defined in PLATO-UVIE-PL-UM-0001 for compression and decompression

• Implement the compression entity header as defined in PLATO-UVIE-PL-UM-0001 for compression and decompression
• small bug fixes

NOTE: The behaviour of the cmp_tool has changed. From now on, the compressed data will be preceded by a header by default. The old behaviour can be achieved with the --no_header option.

Closes #2 (closed)