PhysiBoSS 2.2.3 update

[vincent-noel]

• Update of the libMaBoSS installation script : Using latest MaBoSS, now in the PhysiBoSS folder.
• Introduced steepness : A parameter controlling the hill coefficient used in the smoothed output mapping.
• Introduced use_for_dead : A parameter specifying if the mapping is active after cell death.
• Added PhysiBoSS_DOI and PhysiBoSS_URL, to be used in add_software_citation.
• Updated CITATION.TXT to add PhysiBoSS 2.0 reference.
• Added MaBoSS license in license folder.
• Producing a proper error if BND/CFG file don't exist.

This is what I have for now for the next PhysiCell update. I might add new things before the update happens.