Correctly document additional `ci_method`s (like `"cpi"`)

[jemus42]

Currently, FeatureImportanceMethod implements the $importance() method to aggregate importances scores by feature and add CIs via the chosen method (if any):

https://github.com/jemus42/xplainfi/blob/88c6ba77bdf9b940e1c718bffddd96332ae9f5e4/R/FeatureImportanceMeasure.R#L155-L217

The supported methods can vary between subclasses, which is why there is a "registry" for supported methods https://github.com/jemus42/xplainfi/blob/88c6ba77bdf9b940e1c718bffddd96332ae9f5e4/R/FeatureImportanceMeasure.R#L379 and they are implemented as private methods with .importance_{method} naming scheme.

To implement CPI for the PerturbationImportance method alone, It's added to the registry on initialization https://github.com/jemus42/xplainfi/blob/88c6ba77bdf9b940e1c718bffddd96332ae9f5e4/R/PerturbationImportance.R#L65 and as a private method https://github.com/jemus42/xplainfi/blob/88c6ba77bdf9b940e1c718bffddd96332ae9f5e4/R/PerturbationImportance.R#L238

The issue now is that ci_method = "cpi" does work, but it takes additional arguments (test, B) and I don't know where to document them, since I can only document $importance() once in FeatureImportanceMethod.

[jemus42]

• Add $importance() methon in PerturbationImportance
• Document cpi there and additional args
• Call super$importance()
• Implement ci methods as standalone functions, can also be exported maybe. Need to be careful because some need resampling information (accessed via self), so the relevant args may need to be passed explicitly or the (instantiated) resampling itself could be passed.