Aitchison distance, clr transformation, and their robust variants

[antagomir]

This PR solves #433 and provides:

New transformation to vegan::decostand:

• CLR transformation ("clr")
• robust CLR transformation ("rclr")

New distance to vegan::vegdist:

• Aitchison distance ("aitchison")
• robust Aitchison distance ("aitchison_robust")

These options are documented, references and unit tests have been provided. The automated build & checks seem to go through.

Remaining questions:

• Are contributors listed in DESCRIPTION (I could add this to the PR)? The newer formats often include authors with different roles in this file (see here), like "creator", "author", "contributor" etc. But the current system does not use this format so I am not sure. Can also add an update to the full author listing format, if this is desirable and could fit in the same PR.
• NEWS file update? Would require opening up a new section with an updated version number.
• Any other issues, happy to fix!

Also thanks to the colleagues Felix Ernst & Tuomas Borman for contributions to the code via the mia package, from which these were transferred and modified.

[jarioksa]

This looks OK for a merge. There is one bug, but I'll fix that after merge (vegdist.R needs {} after if() when you have more than one statement – R is not python where indentation defines the block structure). (Or you can do it if you wish: I'll delay merge in case you want to make the fix.)

As to authors in DESCRIPTION: I'll re-write it to use Authors@R and then when we can update author information more easily. We have discussed this with @gavinsimpson , but we have just been lazy for the change. Wait for that.

Thanks for the contribution!

[antagomir]

Cool, I added curly brackets now, I hope I didn't miss any. Looking fwd to seeing this in CRAN and taking into use in other pkgs - !