Allow rank-deficient fixed-effects model matrices

• Extend the MatrixTerm type with a piv vector and rank integer scalar. Use a pivoted Cholesky factorization to determine the computational rank of X'X.

• fixef gains an additional, logical argument pivoted that defaults to true.

• coef always returns the fixed-effects coefficients in the original order. Coefficients for columns found to be linearly dependent are set to -0.0 so that X*coef(m) is the fitted value that would be obtained via projection.

• add the 1-argument methods for StatsBase.predict (the result is equivalent to StatsBase.fitted).