Add healpix to hips conversion method

[adonath]

This PR adds the healpix to hips conversion functionality. It is exposed via the method healpix_to_hips. I added a simple test using a healpix image filled with np.arange() in the nested scheme and compared the sum of the indices to manually created reference values. I also added an environment.yml defining a hips-dev environment.

[coveralls]

Coverage increased (+0.2%) to 96.301% when pulling 674e3e6a6bf89ede434f95552c3ce42c9213a474 on adonath:healpix_to_hips_conversion into 8d6078ffb08f2fd1e63e54b305b5b336db8f535c on hipspy:master.

[adonath]

@cdeil OK to add the high level docs example later, once the support for color image is added?

[cdeil]

Sure

[cdeil]

@adonath - Thanks!