Use `fastobo` to load ontologies from OBO files

[althonos]

Hi !

This PR (finally) adds a native OBO parser using the fastobo package. This is very much WIP since there are no actual semantics for OBO to OBO Graphs translation, and the translation is in early development.

I added it in a way fastobo becomes a mandatory dependency, which is not optimal on platforms other than Linux and OSX x86-64 since it requires a Rust toolchain to be available on the target machine; I could make it optional as well if you think this is better.

[cmungall]

How would the optional option work? Import as-needed if it encounters a .obo file?

I think I'm tending towards keeping it simple, not sure how many Windows users we have, if any

[dougli1sqrd]

So this requires rust on the machine that is running fastobo? Does pip install this locally in a virtual environment? Like, will someone have to separately install rust before running this?

[althonos]

So this requires rust on the machine that is running fastobo?

Not at runtime, you don't need Rust once the fastobo package is installed.

Does pip install this locally in a virtual environment?

This is like any Python library, you can use fancy stuff like venv or just do a site install with pip.

Like, will someone have to separately install rust before running this?

If they are running an x86_64 Linux or OSX, no, because I provide pre-built versions. If they are not, they need a Rust toolchain at compile-time, but once the module is compiled the toolchain can be uninstalled.