move to c++17 and start a coding standards doc

[garyjg]

I did some checking, including with Chris about the Vortex target (embedded i386 PC104 running Ubuntu Bionic), and all of the current target platforms for NIDAS already have GCC versions which support C++17. This includes the Debian buster armhf currently on the Raspberry Pis, in the process of being replaced with aarch64 bookworm. However... unfortunately compiling with -std=c++17 on the NIDAS buster branch fails because of all the dynamic exception specifiers. Those were removed on the master branch, so once master and buster are merged (#16) it should be feasible to move to C++17. Among other things, this means boost::filesystem can be replaced with std::filesystem.

Also, there should be a document in the source stating that the target C++ standard is C++17, along with any other coding standards important enough to document.