Add Precision at R to precision at k

[michaelbornholdt]

This update should be done soon after #63 since it is fast to implement and important for me to use for my thesis.

I will simply add the option to do precision at R which will set k to the number of correct neighbors such that the precision can always be between 0 and 1 no matter how many MOA replicas exist for a given class.

[michaelbornholdt]

Precision@R function lives in my fork right now