Implement problem interface

[cmhyett]

• Reference (and link to) the issue/feature this addresses
  Works to solve Solidify some interface for create_system #3
• Give a short description of your changes
• All new sub-routines are unit tested
• New/amended functionality is covered by integration test(s).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (611b5c1) to head (2cf3ba1).
Report is 2 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff            @@
##              main       #13   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           17        17           
=========================================
  Hits            17        17           

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[cmhyett]

• Lots of progress, just need to check the equations and ensure compatibility with ODE solvers now.