Support RGB in HEALpix -> HiPS

[cdeil]

This PR addresses #136 and adds RGB image support for the HEALPix -> HiPS conversion.

I'll try to add a docs example that shows that it really works before merging this in.

[coveralls]

Coverage decreased (-0.3%) to 95.322% when pulling da470d1 on cdeil:healpix-rgb into 3719ae4 on hipspy:master.

[cdeil]

@tboch - Could you please have a look at the docs example here?

Is it possible to show HiPS in Aladin Lite without Allsky.fits using some options?
Or do I have to generate it to make this work?

For Aladin Desktop I also don't manage to load the HiPS, if I just pass the folder name.

Probably there's still bugs and missing information ... ?

[tboch]

@cdeil: currently, this can not be displayed in Aladin Lite for 2 reasons : AL currently ony supports JPEG/PNG tiles and also requires the presence of Allsky (work in progress to get rid of the latter limitation).

I did not manage either to visualize the generated HiPS in Aladin Desktop, I'll check with Pierre tomorrow, I'm sure there must be a way.

[cdeil]

I'll change the example to PNG and also generate an AllSky. Then it should work in Aladin Lite.
The JS code to load the files from local disk is OK?

[tboch]

The JS code to load the files from local disk is OK?

almost. It should be:

aladin.setImageSurvey(
        aladin.createImageSurvey(
            'test123', 'test123', '.', 'galactic', 1, {imgFormat: 'png'}
        )
    );

[cdeil]

AL currently ony supports JPEG/PNG tiles and also requires the presence of Allsky (work in progress to get rid of the latter limitation).

That would be nice to make it work without Allsky. Also, if possible, making it work for any order, not just >=3 would be nice. I'll try to change to order=3 for the test and see if that works.

[cdeil]

I've continued with this a bit in 866748a .

I think it would be better to have the order as parameter from the start and to pass it in to all the functions, instead of inferring it from the HEALPix array size and tile width deep down in healpix_to_hips_tile:
https://github.com/hipspy/hips/pull/141/files#diff-4de2f89531f836afdfd881e73aea8632R78
I'll try that now.

[cdeil]

With 6df49aa we now have for the first time generated some HiPS that Aladin Lite can load:

However the tiles clearly aren't filled correctly yet. Could be a bug in many places, e.g. the allsky tile handling or the hipsgen part. @tboch - If you have time to have a look, please do. Any suggestions / code review highly welcome. Otherwise I'll try to come back to this and figure it out / finish it up in the coming days.