Fixes multistate structural analysis time storing & reading

[IAlibay]

Changes the datatype for storing time to float, and adapts test to pass for new upstream fix.

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[github-actions]

No API break detected ✅

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.84%. Comparing base (1c2d40d) to head (8c1704c).
Report is 118 commits behind head on main.

Files with missing lines	Patch %	Lines
openfe/tests/protocols/test_openmm_rfe_slow.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1114      +/-   ##
==========================================
+ Coverage   90.75%   91.84%   +1.08%     
==========================================
  Files         142      142              
  Lines       10528    10527       -1     
==========================================
+ Hits         9555     9668     +113     
+ Misses        973      859     -114     

Flag	Coverage Δ
fast-tests	91.84% <0.00%> (+1.08%)	⬆️

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[atravitz]

lgtm

[IAlibay]

Thanks @atravitz - I'll go ahead with the merge so I can update the other branch.