Sector class - v.0

Implementation of the sector class. I still have to finish to implement
the coordinate field and the secondary structure field.
Change location field to membrane field

I don’t know where to find the location but I can tell if the word
membrane appears in the keywords of the pdb file.
Bugfix: multiple uniprot accession numbers

If the pdb file contains a protein complex, there might be more than
one uniprot accession number. The uniprot field is now a cell array
containing one or more accession number.
No support for protein complex yet !!!

File is not ready yet to use, still have to implement the coordinate
part and the protein complex support.