Improve tile I/O

[cdeil]

This pull request:

• Introduces hips.tile.io as the place where the default HiPS URL and filename are computed
• Exposes the default URL in a (hopefully) slightly simpler and better way on the existing HipsTileMeta and HipsSurveyProperties classes.
• Change tile I/O to store raw bytes and use that byte buffer in read / write calls, delaying the extraction of the pixels as a Numpy array to a tile.data property access.
• Activate test that shows that FITS / JPG and PNG tile read / write now works and roundtrips.
• Misc code / test / docs cleanup

[coveralls]

Coverage decreased (-2.5%) to 94.221% when pulling 495485c on cdeil:rgb-tile-io into 91f9eea on hipspy:master.

[coveralls]

Coverage decreased (-0.06%) to 96.632% when pulling fdfd672 on cdeil:rgb-tile-io into 91f9eea on hipspy:master.

[coveralls]

Coverage decreased (-2.7%) to 93.97% when pulling 36c178c on cdeil:rgb-tile-io into 91f9eea on hipspy:master.