better association handling

[longemen3000]

related to #52 .
the current situation is the following:
we have a damped sucessive substitution solver for the association fractions. the only way to control the current output of this is by passing a absolutetolerance=1e-12 keyword
as of 0.2.6, we solve analytically when there is one site (giving a lot of speed up on pure models)
The iteration procedure has a damping factor of 0.5 and (as 0.2.8) does 1000 iterations before giving up.
Maybe we can create an struct that can hold more information about the iteration procedure in addition to the tolerance, like iterations, damping factor, something like:

struct AssocOptions
  iterations::Int
  dampingfactor::Float64
  absolutetolerance::Float64
end

Another way to to this would be by calculating the max amount of iterations automatically, maybe 500*number_of_sites ? or a quadratic relation?