New module for accessing the HSC-SSP archive

[ruizca]

This adds a new service for accessing data from the Hyper Suprime-Cam Subaru Strategic Program. Examples of use are included in the documentation (docs/hsc/hsc.rst).

Note that you need a valid account in the HSC archive for using this module (see https://hsc-release.mtk.nao.ac.jp/datasearch/new_user/new). Due to this fact, I'm quite lost on how to implement remote tests. Any comments or suggestions about this (or any other issue you find) are most welcome.