Add Missing Data Imputation Methods

[antoinecarme]

PyAF needs some kind of missing data imputation.

Different methods are available :

1. Do Nothing
2. Discard rows with missing data
3. Interpolate with available data
4. Mean/Median (signal only)
5. Constant value (signal only)
6. Previous Value (signal only)
7. Other ??

These methods apply to time and signal.