Decompression code has been restructured.

The new layout should make easier to decompress many files into different HDUs of one FITS file.
Code restructured to allow the compression of differenced data

So far the code still compresses only pointings, but it should be easier now to include differenced data.
The program is now able to compress differenced data.

The ability to decompress the data is not implemented yet.
Small error (detected using Valgrind) fixed.