Implemented Cholesky decompositions

Implemented three kinds of Cholesky decompositon:

• cholesky, which performs a full Cholesky decomposition in
  O(n^3)
  .
• incomplete_cholesky, which performs an approximate Cholesky decomposition in
  O(n)
• sparse_cholesky, which performs a full Cholesky decomposition somwhere between
  O(n^3)
  and
  O(n)
  , depending on the sparsity structure.