EvaluationResult of Negative Binomial number test should return variance / alpha

[pabloitu]

The evaluation function https://github.com/SCECcode/pycsep/blob/5f84ea97101de0439deb1e3f5c383874c7bb3801/csep/core/binomial_evaluations.py#L33 requires a variance parameter. However, this is not stored in the resulting EvaluationResult. To plot this negative binomial consistency test, the variance parameter is required again, which impedes the EvaluationResult to be a standalone object without the function arguments that created it. I propose to change https://github.com/SCECcode/pycsep/blob/5f84ea97101de0439deb1e3f5c383874c7bb3801/csep/core/binomial_evaluations.py#L68 to

    result.test_distribution = ('negative_binomial', fore_cnt, variance)

This would imply to remove the variance argument from the plot_consistency_test signature and retrieve it from the EvaluationResult argument the plot function has.