Add description and images for ST_EnvelopesIntersect

[gpetit]

Add description and images for ST_EnvelopesIntersect

Todo : add some unit test for this function

We should check if this function is really SFS

See #108

[agouge]

Apparently these are all equivalent:

ST_EnvelopesIntersect(geomA, geomB);
geomA && geomB;
ST_Intersects(ST_Envelope(geomA), ST_Envelope(geomB));

And this is definitely not SFS.
Now the question is whether we remove this function from H2GIS. @nicolas-f

[agouge]

The best idea is probably just to move it to h2spatial-ext.

[gpetit]

The function is already developped, so I agree with @agouge . We can just move it to h2spatial-ext.
If it's OK for you, do I have to change something in the .md file ?

[agouge]

Yes, we need to move it. Once it is moved, you'll have to modify the md to fix the navigation.

[agouge]

I merged 3b9fe6b but I'm abandoning 7482d8a for now until #319 is fixed.