Find iqtree executable

[trvrb]

Context

IQ-TREE 2.1.2 offers some significance performance improvement specifically for SARS-CoV-2 (http://www.iqtree.org/). However, it slightly changes some of it's command line options and changes the name of the executable from iqtree to iqtree2.

Description

I'd like augur tree to use iqtree2 when it's available but to fall back to iqtree when it's not. This should offer an upgrade path to existing users. Currently, FastTree and RAxML use a find_executable function to determine which binary to call (https://github.com/nextstrain/augur/blob/master/augur/tree.py#L90, https://github.com/nextstrain/augur/blob/master/augur/tree.py#L40). We should just add a similar lookup to IQ-TREE.

[trvrb]

I just elevated this to "Product backlog" with the thought that providing iqtree2 to users will be a significant performance improvement for little effort. But it can easily be shifted back to "icebox" if desired.

[huddlej]

IQ-TREE 2.1.2 is now in Bioconda, as of today. The Bioconda version renames the binary to iqtree, for backwards compatibility, and keeps the newer iqtree2 binary, too.

We should still implement the change proposed in this issue, though, for consistency across our tree builders.

[huddlej]

@benjaminotter, can you add the call to find_executable that Trevor describes above for the build_iqtree function? This will work like the existing call for the build_raxml function.

You should be able to test that this function works by installing the latest IQ-TREE 2 release on your path as iqtree2 first and then also as iqtree.