Flatten `tutorials/` into a single-level directory

[alexismhill3]

Originally I was keeping the inputs to the tutorial in a nested subdirectory (tutorials/data) but this is unnecessary, especially since there are only 3 data files. I've flattened tutorials/ so that everything (inputs and notebook) is on the same level, which should make running the tutorial a bit more straightforward, especially if the user is moving files around.

I also added a line to the tutorial instructions that reminds the user to copy over all files (not just the python notebook) if running the tutorial from another working directory.

Resolves #203

[Thomieh73]

I agree with your thinking here. :-)