Add type class instance for Sample equivalence

[dlindhol]

We want to be able to check Sample equivalence in tests. It seems worthy to provide that concept outside of the testing code, e.g. as a type class instance in the Sample companion object. Not sure if this should be a cats.Eq or a scala.math.Equiv.

We effectively have the latter for Samples via a PartialOrdering[Sample] defined in LatisOrdering. That uses only domain values to determine the ordering of samples.

First we need to carefully define what we mean by Sample equivalence and outline use cases (especially as they relate to sample ordering). We presumably want to use the concept of "scientific equivalence" here which can be forgiving of value types and precision. It may be sufficient to extract doubles from numeric variables or strings from text variables. Not sure what to do about precision. It's one thing to deal with in tests but another if we start using this in other operations.