Fix NaN occurence in Mitochondria

Modify MitochondriaAssignmentProcess.cpp and Mitochondria.em. NaN
occurence is fixed.
Modify IsotonicContractionAssignmentProcess.cpp

Modify IsotonicContractionAssignmentProcess.cpp to avoid NaN generation
when X value is low.
Adjust kuzumoto_et_al_2007_2.5Hz.Anoxya.em to the new Cytoplasm model