Added functions to extract expected probabilities

[Kaladani]

Description

Added invisible functions (not exported to namespace and only accessible via RSiena:::functionName()) that extract expected change probabilities for the observed data (RSiena:::expectedChangeProbabilities), as well as a first version of RSiena:::expectedChangeDynamics() to extract the change probabilities for each ministep. Both are provisionally stored in a single sienaMargins.r together with helper functions softmax() and calculateChoiceProbability(). All functions are based on code from sienaRI.r and sienaRIDynamics.r. They are meant to provide a starting point and could potentially have similar memory leakage problems as the original sienaRI-functions.

Currently, only expectedChangeProbabilities() does actually calculate all possible choices (for the first potential mini-steps in the observed network), while expectedChangeDynamics() does only calculate the choice probabilities for the focal actor in each simulated mini-step. Being able to use the mini-steps actually done in the original fit of the model might also be preferable in the future.

The rate function is ignored for now, the probabilities are all conditional on choosing the respective focal actor.

Update 2024-06-10: I have also set up a testthat infrastructure in the branch and the corresponding test file for sienaMargins.R runs succesfully.

Checklist:

Checks

• If possible, I have added tests that prove my fix is effective or that my feature works. See tests/testthat/test-sienaMargins.R
• The package builds on my OS without issues
• My changes generate no new warnings

Documentation

• not made for release yet