Convert to R package

[xec-cm]

Hi @FrederickHuangLin,

First congratulations for ANCOM! This is a very useful and robust tool.

I am working on the development of some R packages for metagenomic data analysis and some functions depend on ANCOM. To improve the integration of ANCOM with other tools I have made some changes to the repository to convert it into an R package.

1. Move the ancom_v2.R to R folder and add function documentation (from you repor README)
2. Add *-sample-metadata.tsv and *-table-taxa.tsv as a package data. This helps to perform code tests. See data-raw floder.
3. Remove out and __scripts folders
4. Move images volder to inst/extdata folder
5. Add R-CMD-check of the package via github actions. Checks are performed in macOS and ubuntu in multiple R versions.
6. README simplification. It would be nice to move specific examples to vignetts in the future.

If you want to reverse or incorporate any changes I can help you. And if in the future you want to submit the package to bioconductor that would be great (I can also help you).

Bests

Francesc