Internal Dataframes index refactoring

[antoinecarme]

PyAF uses a lot of dataframes internally for each step of the signal decomposition process.

Profiling shows that not all these dataframes are indexed properly, leading sometimes to performance issues (merge, re-indexing takes time).

Revisit all the code and use the proper index for each dataframe.

Release date : 2022-07-14

[antoinecarme]

Ensure that PyAF is and remains Green #176

[antoinecarme]

Closing.