ENH: CONESTA 

- Use curent mu as stoping criterium intsead of function.mu_opt(self.eps).
- Some refactorization of the algo: simplify variables names and suffix optional returned
information variable with "_" to  spot optional variables ex: mu_, t_, f_ are.
- Avoid unnecessary computation of the gap within CONESTA get it from the last FISTA
iteration.
- Same modification for StaticCONESTA