Implement `JointProbabilitiesEstimator`

[kahaaga]

This paper discusses different ways of getting probabilities from counts for two variables. We should formalise this using the abstract JointProbabilitiesEstimator <: ProbabilitiesEstimator type, and let subtypes of it dictate how counts are translated into probabilities probabilities(estimator::JointProbabilitiesEstimator, input_data...). Currently, we just implicitly use RelativeAmount.

[Datseris]

What does two variables mean here? Two input datasets? I thought this was for Causality tools.jl

[kahaaga]

What does two variables mean here? Two input datasets? I thought this was for Causality tools.jl

Two input datasets, yes. And this is the issue tracker for CausalityTools, so it should be here :D

[Datseris]

Sorry, I completely missed looking at the package name :D

[Datseris]

with the millions of estimators in complexitymeasures.jl it's easy to get lost ;)