Add default value for piecewise loading

[teubert]

Add the ability to send a loading profile that is one longer than the times. The last value is treated as the "default value"- the load that will be applied once the end of the profile is met.

Also added some input validation and associated tests

[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 #82 (7ccf86b) into dev (a421b92) will decrease coverage by 0.04%. The diff coverage is 54.54%.

@@            Coverage Diff             @@
##              dev      #82      +/-   ##
==========================================
- Coverage   84.26%   84.23%   -0.04%     
==========================================
  Files          99       99              
  Lines       10167    10178      +11     
==========================================
+ Hits         8567     8573       +6     
- Misses       1600     1605       +5     

Files Changed	Coverage Δ
src/progpy/loading/piecewise.py	72.22% <54.54%> (-27.78%)	:arrow_down: