Include MLE estimation of tau_0 and sigma_2

[lionelvoirol]

• Added fcts ar1_sim_geneva(), n_ll_unevenly_spaced_ar1() and ar1fit_m()l in file tsmodel.py
• Created 2 tests to ensure MLE estimation works fine with evenly and unevenly spaced AR1.
• Added time_increments() to tsmodel.py, together with tests
• eventually, the surrogate method ar1sim will point to ar1_sim_geneva(), and the current one will be relabeled ar1old for benchmarking purposes.
• For now functionalities are unaccessible from the user API ; waiting on #228 to be completed (in stages)