Marker2sequence requires that the genes have a type, this commit fixes this
Let's check that there is a description associated to the gene before adding it to the model
Implement the conversion of the protein-protein interaction model

The PPI information are retrieved from the EBI website, intact.txt file.
Interaction are going both way
Fix protein URI to use uniprot's
Add the code to do the downloading of the lastest files
Fix PPI and the use of the URIs
Rename the PPI class into PPIAction

- Follows the structure applied so far
- Refactor the class to separate the download and main methods, following the structure applied in the other action class
Add the --ppi argument

The user can retrieve the protein-protein information using this argument.
Add LICENSE file and header
Update file name
Update the --ppi function

- Update file name
- Make use of the PPI_files folder
- Handle situation where the cell contains '-' instead of a protein identifier