Feature: save_freq of 0 saves every pt

[teubert]

Update save_freq to allow 0, a save_freq of 0 now saves every point. Updated next_state for auto to ensure it doesn't get stuck on a step size of 0

[github-actions[bot]]

Thank you for opening this PR. Each PR into dev requires a code review. For the code review, look at the following:

• [ ] Reviewer (someone other than author) should look for bugs, efficiency, readability, testing, and coverage in examples (if relevant).
• [ ] Ensure that each PR adding a new feature should include a test verifying that feature.
• [ ] All errors from static analysis must be resolved.
• [ ] Review the test coverage reports (if there is a change) - will be added as comment on PR if there is a change
• [ ] Review the software benchmarking results (if there is a change) - will be added as comment on PR
• [ ] Any added dependencies are included in requirements.txt, setup.py, and dev_guide.rst (this document)
• [ ] All warnings from static analysis must be reviewed and resolved - if deemed appropriate.

[codecov-commenter]

Codecov Report

Merging #68 (1a44508) into dev (334b190) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev      #68   +/-   ##
=======================================
  Coverage   84.17%   84.18%           
=======================================
  Files          99       99           
  Lines       10106    10111    +5     
=======================================
+ Hits         8507     8512    +5     
  Misses       1599     1599           

Impacted Files	Coverage Δ
src/progpy/prognostics_model.py	88.52% <100.00%> (+0.02%)	:arrow_up:
tests/test_base_models.py	97.61% <100.00%> (+0.01%)	:arrow_up: