Harmonize return of model_id for local and async fit

[DrAnaximandre]

Fitting in local returns a string model_id whereas fitting in async returns a unicode string.

Example:

• Expe = Experiment(model) Expe.fit([data],[data_val]) print(Expe.mod_id) returns '7f66b5fdca51fe6fb21f53149e5bba2d'

• Expe.fit_async([data],[data_val]) print(Expe.mod_id) returns u'34285e3cf7ff08eece3db0d83399bf8c'

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We should harmonize the behavior.