Minor quality of life improvements

[michalk8]

Got 3 changes to the API I'd like make:

• make __repr__ and __str__ be the same (or swap them); this means that we don't have to run print(ck) to show the parameters, just ck I'd do this for both kernels and estimators; alternatively we can just swap them
• return self in most of the estimators' methods so that we can chain function calls, e.g. g.fit().predict() or g.compute_schur().compute_macrostates().set_terminal_states().compute_absorption_probabilites()
• improve TermStatesEstimator repr by including #term states there + include #macrotates for GPCCA

[Marius1311]

Sounds good!