Iss12

Hi.

I've implemented PositiveDefiniteMatrixPredicate from this issue - https://github.com/vkostyukov/la4j/issues/12.
The current implementation was taken from your post (reference implementation). Additional condition was also added in order to perform calculation faster, such as (algorithm has to work only on a square matrix with a positive determinant).
Hope, that everything is ok.

Best regards.