Enhance testing of library

[rboston628]

This PR split off from PR #7 , which had become too unwieldly with its many edits.

This PR is focusing specifically to changes in library methods, and the tests to protect those methods.

The following library classes had refactors:

• Splinor: methods for different boundary conditions, split off duplicated coefficient methods
• matrix: there was an error in the inversion process, caught by the new tests, now fixed
• stellar: fixed misspelling the name of famous physicist Max Planck

The following source code had to change in response to changes in the library:

• constants uses the correct spelling of Planck
• SimpleWD uses the correct spelling of Planck

In addition, the following changes were made to source code:

• Isopycnic: fixed a typo in the surface coefficients of $A^\ast$ and $V_g$. Also removed light_speed2 definition, which instead just inherits from Star.
• MESA: changed some ints to size_ts, and updated certain surface expansion methods, which were erroneous.
• Polytrope: removed light_speed2 definition, which instead just inherits from Star
• Star: replaces some string methods with new formatted strings, replaced some ints with size_ts.
• ThrainMode: fixed some finder methods

And now the changes to the tests:

• adds CppCheck to CI/CD
• many tests have had their print-outs suppressed during running tests, to make output mangeable
• matrix: add extra test of tridiagonal inversion
• rootfind: replaces the TS_ASSERT_IS_NAN(x) method with more stable/secure TS_ASSERT(std::isnan(x))
• Splinor: many, many more tests, including using test data functions, and a test to create outputs to "artifacts" folder
• string: tests buffer overflow
• ThrainMode: replaces the TS_ASSERT_IS_NAN(x) method with more stable/secure TS_ASSERT(std::isnan(x))
• ThrainUnites: adds a test of params method