explanation comments, variables fraction coloc

add description of the maths to calculate the fraction (%) coloc
results as per the old Colcoalization Threshold plugin.
Doesn't work yet, just a commit to tidy up inbetween

Here we realize that Manders;' coeffiicents a e just another special
case of split fraction coefficients of coloc.

Conflicts:
src/main/java/algorithms/MandersColocalization.java