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.

merged

mentioned in commit ff7473a9