fix score test function

[jr-leary7]

• currently scoreTestGEE() does not properly account for working correlation structure of the full model
• need to recompute the working variance-covariance matrix for each subject under the desired correlation structure, then sum up contributions to the score vector $\mathbf{U}$ and its variance $\mathbf{V}$
• can then correctly compute the test statistic and accompanying p-value