Add type hints to preprocessing functions

[PierreGtch]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 86.89%. Comparing base (095e70c) to head (8074c08).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #600   +/-   ##
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  Coverage   86.89%   86.89%           
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  Files          65       65           
  Lines        5759     5768    +9     
=======================================
+ Hits         5004     5012    +8     
- Misses        755      756    +1     

[agramfort]

diff looks good but I feel we here need to add mypy support to check that it's actually correct.

+1 for merge and add mypy CI job in another PR

[bruAristimunha]

Thank you @PierreGtch