Proposal to remove pystring dependency

[KevinJW]

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