General tasks for V0.16

[PierreRaybaut]

Update documentation to include the following new features:

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

Then, update the following sections:

• [ ] Key features

Add @pytest.mark.validation marked unit tests for:

• [ ] Signal operation features
• [ ] Signal processing features
• [ ] Signal computing features
• [ ] Image operation features
• [ ] Image processing features
• [ ] Image computing features

And finally:

• [ ] Update the translations
• [ ] Update the change log:
  • [ ] Add all changes, commit by commit
  • [ ] 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)