Add a converter for converting seconds in float to timedelta

[sbethur]

• This contribution adheres to CONTRIBUTING.md.

- [ ] I've updated CHANGELOG.md if applicable.

• I've added tests applicable for this pull request

What does this Pull Request accomplish?

Add a converter for converting seconds in float to timedelta.

List issues fixed by this Pull Request below, if any.

Required for fixing #1397

What testing has been done?

Added new unit tests. Tests will be run by CI.

[codecov]

Codecov Report

Merging #1434 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
+ Coverage   91.90%   91.92%   +0.02%     
==========================================
  Files          20       20              
  Lines        3642     3655      +13     
==========================================
+ Hits         3347     3360      +13     
  Misses        295      295              

Flag	Coverage Δ
#codegenunittests	88.24% <ø> (ø)
#nifakeunittests	96.40% <100.00%> (+0.03%)	⬆️
#nimodinstunittests	95.37% <ø> (ø)
#nitclkunittests	95.45% <ø> (ø)

Impacted Files	Coverage Δ
generated/nifake/nifake/_converters.py	96.67% <100.00%> (+0.06%)	⬆️
generated/nifake/nifake/session.py	97.69% <100.00%> (+0.02%)	⬆️

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