General tasks for V0.16

[PierreRaybaut]

Update documentation to include the following new features:

• [x] feature/constant-operations
• [x] feature/exponential
• [x] feature/fft_bode
• [x] feature/flip-x-axis
• [x] feature/high-low-pass-filters
• [x] feature/impulsion-scalars
• [x] feature/new-signal-fitting
• [x] feature/new-signals
• [x] feature/normalisation
• [x] feature/offset-operation
• [x] feature/read-mat-files
• [x] feature/signal-scalars
• [x] feature/signal_sqrt-pow
• [x] feature/windowing

Then, update the following sections:

• [x] Key features

And finally:

• [x] Update the translations
• [x] Update the change log:
  • [x] Add all changes, commit by commit
  • [x] Add an entry regarding this bug fix in PlotPy which fixes an unexpected behavior in DataLab (annotations visibility was not saved/restored when switching from/to the separate view)