Rationalize CMake and switch to C++17 to accommodate MFEM

[IdoAkkerman]

This PR does three related things -- in order of impact/controversialness(sic):

1. It rationalized/simplifies the CMake routines.

• In the old routines the MFEM library defaulted to dependencies/mfem even if another mfem was provided
• The mfem is imported as a packages, this simplifies things
• Examples and tests are handled in there own directory, this simplifies the routines

2. Switched to c++23 to accommodate MFEM

• This created issues with std
• Removed using namespace std from the library and examples

3. Fixed some other (pedantic) compiler errors

• Added some override statements
• Made a constructor virtual
• Separate delete statement for different variables
• Corrected a string comparison

[IdoAkkerman]

This PR address issues also tackled in PR #318

[ckendrick]

Thanks for making these changes! This looks good to me and works as expected.