Make pyssht an installable package

[EiffL]

In its present form, pyssht is not a proper python package. I have modified the setup.py and moved the source files to an isolated pyssht directory so that it can now be installed easily with the following command:

python setup.py install --user

It shouldn't change the previous behavior, you can still compile the files with python setup.py build_ext --inplace command.

[EiffL]

Hi @jasonmcewen what do you think of this PR ? If you like it and also approve astro-informatics/massmappy#1 people can install massmappy in one command

[jasonmcewen]

@EiffL The public version of ssht was not updated with our recent local updates. I've just pushed an update to the public version now. This seems to have caused a merge conflict with pyssht.pyx. Could you take a look at the conflicts when you get a chance? When they're resolved we can merge the PR.

[EiffL]

Awesome, thanks @jasonmcewen . I'll update my fork with your updates

[mdavezac]

Thanks, @EiffL. We've actually gone one further now. pyssht is now on pypi, which means it can be installed with pip install pyssht.

We've decided to go fully open-source very soon, so we will be archiving the astro-informatics/ssht and making the currently private repo public under the name... /astro-informatics/ssht. As a result you may find your fork no longer leads here. Thanks again.