Differentiate duration and datetime

[aricohen93]

Description

Differentiate results of type duration and datetime.

Identified problem:

When applying the dates pipeline in batch, we obtain in the same column of the result dataframe objects of differnt data type.
Also, the to_datetime() method of a RelativeDate class returns either datetime or duration types depending if note_datetime is given

Some ideas:

• save spans into different keys
• replace to_datetime() by to_pendulum() or maybe to_datetime should be a wraper of to_duration() and to_absolute()

Checklist

• If this PR is a bug fix, the bug is documented in the test suite.
• Changes were documented in the changelog (pending section).
• If necessary, changes were made to the documentation (eg new pipeline).

[codecov-commenter]

Codecov Report

Patch coverage: 60.52% and project coverage change: -0.26 ⚠️

Comparison is base (74fa4c7) 94.06% compared to head (35815ad) 93.80%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   94.06%   93.80%   -0.26%     
==========================================
  Files         172      172              
  Lines        5085     5103      +18     
==========================================
+ Hits         4783     4787       +4     
- Misses        302      316      +14     

Impacted Files	Coverage Δ
edsnlp/pipelines/misc/dates/models.py	89.24% <59.45%> (-8.62%)	⬇️
edsnlp/pipelines/qualifiers/history/history.py	94.51% <100.00%> (ø)

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[percevalw]

I've rebased on master, moved durations to a new duration span group (this one maybe a breaking change !) and added an option to assign a shared label date (except for durations)
I feel like having absolute and relative labels for dates was a bit unnecessary given their shared API. What's your opinion on this @Aremaki ?

[percevalw]

Closing as this was merged in #213