Import and tidy ADMIXTURE output

[dramanica]

We want to have a function that allows us to import the output from ADMIXTURE. The expected path would be:

1. generate a BED from a gen_tibble (after filtering/clumping for LD)
2. run ADMIXTURE in bash as usual
3. in R, use a simple gt_import_admixture which reads in the admixture files and creates a sensible structure, with class gt_admixture. We might want to consider a super-class gt_cluster which would then allow us to recycle tidiers and autoplot for all clustering approaches.
4. Operate with that object via tidy, augment and autoplot
   Things to do:

• select a simple dataset that we can play with. Ideally the same dataset that we will use for other analyses. Currently we have a dataset on anolis lizard that could work in inst/extdata/anolis. It is based on the tutorial:
  https://connor-french.github.io/intro-pop-structure-r/
  There is a .Rmd that replicates the first part of the analysis (DAPC) in /data-raw/adegenet, which also has the code to read and format the dataset as a gen_tibble.
• create a BED from a gen_tibble and then run ADMIXTURE
• include the outputs in inst/extdata for a few clusters
• write the import function (look at the tutorial on lizards as it has some code to do that)
• write tidiers methods (tidy() and augment()) (look at the tutorial on lizards as it has some code to do that)
• write autoplot() method (look at the tutorial on lizards as it has some code to do that)

[dramanica]

Ok, basic functions and an example with LEA are now in the main branch of tidypopgen. Ideally, we should create a run of ADMIXTURE for an example dataset, and write a function to make it easy to import a Q file into the right kind of object for everything else to work.

[dramanica]

This is now fully implemented, we just need to update the vignette.