Add more examples

[Tim-Evans-Seequent]

It would be useful to have a few different examples of real mining data in OMF2 files. Code to write and read the files can be included as well, but isn't necessary.

Example files on their own can be placed into examples/example1.omf for example. If they come with Rust code then put them into a subfolder with examples/example2/main.rs and examples/example2/example2.omf along with any input data.

Update the examples/README.md to provide a brief description of each example.