ENH use scientific number formatting for delta and newton decrement in Cox fitters

[mathurinm]

Hello, When pushing the solver to convergence, both delta and newton_decrement go to zero, so the displayed information looks like this when they becomes smaller than 1e-5:

With the proposed change a useful value is printed all along convergence:

[CamDavidsonPilon]

Love it!

[CamDavidsonPilon]

Thanks @mathurinm