Improve sigmoidal fitting

[aarmey]

Two improvements here. They can maybe be handled in two separate steps:

• There are three sets of cell populations where we should share the sigmoidal parameters {(Treg, Treg mem, Treg naive); (Thelper, Thelper mem, Thelper naive)}
• The different muteins should share the same sigmoidal parameters, and so need these fit together.