Improve HiPS tile I/O

[cdeil]

I would like to propose this solution to address #42 : suppress the warnings from invalid FITS tiles.

Since this is a "known issue" (or "known feature" if you like) for the FITS tiles from CDS, and every user will get one of these warnings per tile they fetch or read on the console, and there's nothing they can / should do about it except to ignore it, I think it is appropriate to suppress these warnings.

I'm not an expert on Python warnings, but I think / hope that this will have little other effect than suppressing the warnings we want to suppress, and to keep other warnings / errors intact.

@tboch - Assigning to you for review.

@adl1995 - I've also cleaned up the I/O functions a bit to:

• always use with statements to read / fetch, to avoid leaving open filehandles around
• removed the code duplication in FITS tile read and fetch by always reading into a BytesIO and then doing the "decoding" in a _from_raw_data method.
• Added PNG as a third case and put NotImplementedError for now, and raise ValueError if not one of the three formats we can handle. I don't know yet if the code to handle JPG and PNG will be identical or different, if yes we could combine these two cases again via elif file_format in {'jpg', 'png'}, keeping the else ValueError part to give a better error message in case a format we don't support appears.

[coveralls]

Coverage decreased (-0.6%) to 86.145% when pulling 5651248d2238f893216e98318f511dc8acb88b4b on cdeil:fits-truncated into 7fccd4c50de7776d30450158c2314263f56a8fb7 on hipspy:master.

[cdeil]

@tboch - Can you comment on this PR (or merge it)?

@adl1995 wants to add support / tests for PNG tiles this week, and it's easiest if this PR (or parts of it) go in first.

[cdeil]

@adl1995 - Now that this PR is in, you could do #38 (i.e. fully support JPEG read / write, making raw_data a HipsTile data member of type BytesIO) or implement code and tests for PNG tiles.

Note that the current tile I/O tests combine fetch / read / write. Now that we have @requires_hips_extra I would suggest to use hips-extra for read / write and all tile tests, and to only have one small test for the fetching using @remote_data.