add vignette

refactor plotting, add normalization

add differential association tests

Allow taxa counts on sequence level

Add new helper functions.

- probbaility estimation using the Orlitsky diminishing attenuation
  estimator
- a function to create nice plots showing the DESeq2 counts across
  a variable

Fix mockrobiota downloads

This release brings better metrics for evaluation of

taxa assignments. Now calculates precision, recall and
F1 scores for each taxa level.

Metrics now use phyloseq objects.

Update README.md