Benchmark intel MKL LDLT algorithms.

The intel math kernel library (package ```libmkl-dev```) contains both dense and sparse LDLT decompositions.

The dense version is a LAPACK implementation (dsytrf), must figure out how to link against it.

```
/usr/include/mkl/mkl_lapack.h
```

The sparse version can be interfaced with through the ```<Eigen/PardisoSupport>``` header.

```
/usr/include/mkl/mkl_pardiso.h
```