Unify duration formatting in mhealth data

[rantahar]

Use turn number and unit columns into pandas TimeDelta columns

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (695aff8) 84.90% compared to head (b07d07a) 84.92%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
niimpy/reading/mhealth.py	92.30%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   84.90%   84.92%   +0.01%     
==========================================
  Files          49       49              
  Lines        3213     3217       +4     
==========================================
+ Hits         2728     2732       +4     
  Misses        485      485              

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