Tag MixedModels v0.3.10

- Add a `PLSTwo` type for solving the penalized least squares problem for models with two crossed or nearly-crossed random-effects grouping factors.
- Added tests for `plstwoscalar` and `plstwovector`.  The model fit in the latter case is singular.
- Removed dependencies on the `NunericExtensions` and `NumericFuns` packages.