Boundaries of sequences or blocks

We've ignored `phi_0` and `phi_1` (the two ends in Gibbs measure) so far, because the sequence  is usually long enough. However if we analyze multi-species data, the boundaries of blocks cannot be ignored anymore.  We might need to find a way to parameterize duplet rates but I'm not sure if it's the best way, because it introduces more parameters (something similar to "baseline" parameters).