Add WCS property in HipsTile class

[adl1995]

I added a WCS property in HipsTile class. As drawing is going to be a major part of this library, I think this is a useful addition to the class.

[cdeil]

@adl1995 - I don't think this will work. Currently the FITS headers of HiPS tiles in FITS format don't contain a valid WCS that would be useful for drawing. See #17.

[coveralls]

Coverage decreased (-0.2%) to 80.252% when pulling c5cd3e6 on adl1995:tiles.wcs into 32db7fb on hipspy:master.

[adl1995]

But, wouldn't we still require this later on?

[cdeil]

@adl1995 - Not necessarily, we can draw the tile without having a WCS for the tile itself.

It would be nice to have though, since if we have a WCS for the tile we can just call reproject as an alternative precise drawing method, as mentioned in #17.

There's nothing you can do here for now, I'd suggest to just close this PR without merging.
Maybe in the future, Thomas will provide code to compute the tile WCS, or maybe we won't go that way.