Create a lower triangular matrix from a vector
stm_tril_from_vector.Rdstm_tril_from_vector() generates a lower triangular matrix from
a given vector. The function calls another matrix function to generate
the matrix.
stm_tril_from_vector.Rdstm_tril_from_vector() generates a lower triangular matrix from
a given vector. The function calls another matrix function to generate
the matrix.