Update stats API to make options last non-output parameter

[jeromekelleher]

We're pretty consistent elsewhere on this pattern of having options be the last param non output param. When we're documenting the stats API, we need to update this, so we have eg

typedef int one_way_weighted_method(const tsk_treeseq_t *self, tsk_size_t num_weights,
    const double *weights, tsk_size_t num_windows, const double *windows, tsk_flags_t options,
   double *result);