PEP518 build system

[clbarnes]

PEP518 build system allows this to be installed directly from pip
without pre-installing numpy and cython.

[pattonw]

Looks like the most important part in the pyproject.toml defining the build dependencies (numpy, cython, etc.) is already in there.
We probably want to just get rid of the Make file, setup.py and requirements*.txt files, in favor of just using a pyproject.toml.

For easy installation we also need to solve the libboost dependency. If boost is already installed then funlib.segment is pretty easy to use, otherwise the installation errors are not very helpful to people unfamiliar with C dependencies.

We only really use boost for a the disjoint_sets datastructure. It's not a terribly complicated data structure so there must be other implementations we can use instead. Scipy has a pure python implementation which we could use but might be a bit slow. I also found this one written in Cython that we could use. Once boost is replaced it should solve all of the common installation issues

[clbarnes]

Yes, the important bits of this PR were all added independently more recently; this can probably be closed with an issue raised for de-boosting the library.

[clbarnes]

Note that that cython implementation has no license so technically we're not even allowed to look at it, or something? I'm never sure. There's an issue raised about it but it seems abandoned. It also needs the build system specified to be installable. Probably best just to vendorise something inspired by but certainly not copied and pasted from it...