Proposal to remove pystring dependency

Our current usage of pystring falls into 2 camps

String manipulation e.g. split, startswith, endswith, splitlines, lstrip etc
Filesystem interaction pystring::os::*

I would propose that this be carried out in 3 stages:

1) Replace string functionality with code based on our own StringUtils (add functionality where needed). We already have some tests for replacing the split() behaviour for example.
2) Replace filesystem interaction by calls to std::filesystem (C++17)
3) Remove all pystring from CMake

Start by locating the existing pystring uses and separating string manipulation from filesystem interaction. Review StringUtils and its split() tests, then assess the proposed std::filesystem and CMake changes. Done means the three replacement stages are complete and pystring is removed from CMake.

Written by the indexing model from the issue text.