Remove unnecessary dt checks

[MarcBerliner]

Description

I removed a few unnecessary checks for minimum dt values to proceed with the simulation.

I also removed the start_step_offset = 1e-9 heuristic setting. For large t_start values, you can get floating point arithmetic errors like t == t + 1e-9 that cause duplicate t values in the solution. I replaced start_step_offset with np.nextafter(t, np.inf), which prevents these errors by giving the next largest floating point number. cc @valentinsulzer

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• Optimization (back-end change that speeds up the code)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.41%. Comparing base (3898453) to head (5430aca).
Report is 206 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4416      +/-   ##
===========================================
- Coverage    99.42%   99.41%   -0.01%     
===========================================
  Files          292      292              
  Lines        22215    22212       -3     
===========================================
- Hits         22087    22083       -4     
- Misses         128      129       +1     

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[rtimms]

thanks, @MarcBerliner looks good to me!