plm::plm() fixes and extensions

This release corrects a bug in the methods for random effects models fitted using plm() and extends the methods to work with nested random effects models estimated by plm().

• Corrected bug in methods for plm objects estimated by random effects, which occurred when a user-specified clustering variable was at a higher level than the random effects.
• Added support for plm objects with nested random effects (effects = "nested").
• Added additional syntactic options for specifying clustering variable with plm objects. See ?plm.
• Corrected bug in how Wald_test() labeled results when test = "Naive-Fp".