Use std::filesystem for path-like objects

[jaelynlitz]

Currently we use char[]s of PETSC_MAX_PATH_LEN with a suite of hand-rolled tools in utils.h for dealing with paths and determining existence of files. This functionality is in std::filesystem, and in a mostly-compliant C++11-compatible header-only library here.

If we move our C++ standard up to 17 (#55), this functionality is present in std::filesystem which is easier to include as opposed to the third party lib that is linked. What are thoughts on changing the standard?

From @pelesh

Updating to C++17 is preferred to keeping an extra dependency imho. Probably more testing is needed than just building a couple of different ExaGO configurations. I am not sure either is needed, though. All we need here are some very standard string manipulations.

[jaelynlitz]

https://github.com/pnnl/ExaGO/tree/std-filesystem