internal split function for RPMS
split(node, data, e_equ, e_fn = survLm, l_fn, X, weights, strata, clusters,
bin_size, perm_reps, pval)
is an integer label of the node
this is the data
is the estimating equation used to model the data in each node
is the function that should be used to fit the model in each node. The function should
function taking a numeric vector and returning the estimated loss
vector of string names for variables to consider for splitting
the sample weights for each observation
lable of the strata containing the observation
lable of the clusters containing the observation
integer specifying the minimum number of observations each node must contain
integer specifying the number of permuations
numeric p-value used to reject null hypothesis in permutation test
list containing split information