simple LDL' factorization library for sparse matrices

 Suitesparse is a collection of libraries for computations involving
 sparse matrices.
 .
 The LDL library provides routines for sparse LDL' factorization and solving.
 These routines are not terrifically fast (they do not use dense matrix
 kernels), but the code is very short and concise. The purpose is to illustrate
 the algorithms in a very concise and readable manner, primarily for
 educational purposes.