Outdated icosahedron function

[oqilipo]

After updating the gptoolbox I had a conflict with the external icosahedron function, because it's an old version from 2009

The current version from 2016 can be found here: https://github.com/pgagarinov/spheretri

spheretri may be be added as submodule?

Thanks and kind regards

[alecjacobson]

Adding a submodule for the icosahedron seems like overkill. I'll happily accept a pull request for the new function if necessary.

[oqilipo]

Done.

kind regards