Functions related to eta parameter used in optim and kkt checks

Used for gradient of eta. Currently being passed to optim in lmmlasso and used in kkt_check

gr_eta_lasso_fullrank(eta, sigma2, beta, eigenvalues, x, y, nt)

fn_eta_lasso_fullrank(eta, sigma2, beta, eigenvalues, x, y, nt)

Arguments

eta	current estimate of the eta parameter
sigma2	current estimate of the sigma2 parameter
beta	current estimate of the beta parameter including the intercept. this should be of length p+1, where p is the number of variables.
eigenvalues	non-zero eigenvalues of the kinship matrix, or the square of the singular values of the matrix used to construct the kinship matrix
x	input matrix, of dimension n x p; where n is the number of observations and p are the number of predictors.
y	response variable. must be a quantitative variable
nt	total number of observations