variexpr: affected, unaffecteds

[brentp]

• should be able to check sample.affected and sample.unaffected.where affected is a boolean
• add affecteds = seq[Sample] and unaffecteds = seq[Sample] for any .ped file with any affected samples. Then user can write a function to iterator over these and return a boolean or number (yet to support numeric returns).

if duktape supported "fat arrow" functions, this would have a nice API. For now, it will have to do that the user runs a function, iterates over each affected or unaffected and returns a result.