numpy and cython should not be required to be installed before setup.py runs

[tibbetts]

Currently setup.py assumes numpy and cython are already installed, which requires a workaround in build scripts to explicitly install them before running pip.

[riastradh-probcomp]

numpy is missing from setup_requires, but I would expect pip to be able to cope with running setup.py once to discover setup_requires and once more, after it has installed the setup requirements, to build the package. (To investigate after release.)