changed Omega_h::filesystem to std::filesystem

[regpacy]

Closes #167

[cwsmith]

Thank you for the contribution and quick response! Are you an RPI student?

Two minor edits are requested below.

[regpacy]

Hello, I resolved issues that you pointed out! and no I am not student at RPI.

[cwsmith]

Thank you.

If you have more time to put into this, would you please take a look at the CI failures?
https://github.com/SCOREC/omega_h/actions/runs/17946795469/job/51040682205?pr=168#step:10:323
It looks like they are in vtkdiff.cpp and Omega_h_vtk.hpp.

[cwsmith]

/runtests

[github-actions]

Test Result: failure (details)

[cwsmith]

Thanks for the fixes.
A few more spots are in an interface to a third party package:
https://github.com/SCOREC/omega_h/actions/runs/17950828852/job/51049377874#step:3:956

git grep filesystem | grep -v std:: may help locate remaining uses.