Resistance association testing for specific variants

[alimanfoo]

Umbrella issue to review and explore methods for testing association between specific variants and a resistance phenotype. E.g., testing association with a specific known resistance marker, such as Vgsc L995F or copy number at Cyp6aa1. The goal being to propose some functions to implement in malariagen_data.

For genome-wide association testing methods see separate issues #679, #683, #684.