Remove dependency on OTU picking

[JTFouquier]

Qiime2 does not require OTU picking as research has evolved/improved.

Remove the dependency/requirement of an OTU map as an input to ghost-tree. Consider making it an option.

Revisit Jai and Evan's comments on Qiime2 forum and Github about this. See my notes about Q2 development for more information.

[JTFouquier]

I'm still undecided on this. I think there are a lot of benefits to using the otu map. Need to think about it.

[JTFouquier]

Consider using/parsing VSEARCH. Also need to add my own check for OtuMap