Add reference output

[willow-ahrens]

Can we add some reference output in this repo with the directory structure:

binsparse-specification/
                spec/
                                …
                reference/
                                amazon-2008/
                                                amazon-2008.mtx
                                                coo/
                                                                format.json
                                                                amazon-2008.bsp.h5
                                                csr/
                                                                format.json
                                                                amazon-2008.bsp.h5

[willow-ahrens]

This would allow us to read a file and compare it to an unambiguous reference for several different target formats

[willow-ahrens]

@BenBrock

[BenBrock]

I think this is a great idea. I recommend we start with chesapeake, which is a classic "small sparse matrix."

[willow-ahrens]

@ivirshup I think everyone with parsers should add a few reference copies so we can ensure the parsers are interoperable