Cesptral log format

[lorisercole]

I noticed that cepstral_log prints with float string format, which can be annoying when dealing with very big/small numbers: https://github.com/sissaschool/sportran/blob/0467486b2c81e2a90783a820c62e41f5ad489b10/sportran/current/current.py#L393

In the examples of #62, this is the output, which is kind of hard to read

-----------------------------------------------------
  CEPSTRAL ANALYSIS
-----------------------------------------------------
  cutoffK = (P*-1) = 10  (auto, AIC_Kmin = 10, corr_factor =  1.0)
  L_0*   =          18.297017 +/-   0.057597
  S_0*   =   127801909.034038 +/- 7361041.782368
-----------------------------------------------------
  kappa* =           0.000229 +/-   0.000013  Pa*s
-----------------------------------------------------

I propose to use scientific notation whenever numbers are too big/small.