1724 Enable loading of TOML files

[rprospero]

The file loading now goes through four stages:

1. If the file name ends in ".toml", force the user of the TOML parser and fail if it fails
2. Check if the file is a restart file and fail with an error telling the user about the problem if it is
3. Try parsing the file as TOML
4. Parse using the original parser

Closes #1724

[trisyoungs]

Looking good. It's a shame that C++ users have to wait until C++20 to get std::string_view::ends_with(), though. Urfff!