Define precision and recall @ k

[shntnu]

(Stubs for now, so we can add this documentation to code later)

Precision@k can be computed in many different contexts:

1. For each replicate well, the fraction of replicates of the same perturbations, among the k most-similar wells.
2. For each guide, the fraction of guides targeting the same genes, among the k most-similar guides.
3. For each compound (in the context of MOA), the fraction of compounds in the same MOA class, among the k most-similar compounds.

Recall@k is defined similarly e.g. the fraction of all the replicates that are among the k most-similar wells.