Add HessianMatrix and TensorEigenValues

[hanslovsky]

• Use PartialDerivative to generate linear representation of upper
  triangular Hessian matrix
• Add TensorEigenValues class to compute eigenvalues on tensors such as
  the Hessian matrix included in this commit.hessian eigenvalues

This pull request rebases all commits in and closes #29.

[tpietzsch]

thanks @hanslovsky, great work!

[tpietzsch]

@hanslovsky Can you please fix the javadoc

[ERROR] /home/travis/build/imglib/imglib2-algorithm/src/main/java/net/imglib2/algorithm/linalg/matrix/RealCompositeMatrix.java:47: error: reference not found
[ERROR] *         {@link RealMatrix} that reads data from {@link Composite} (non-copy).
[ERROR] ^
[ERROR] /home/travis/build/imglib/imglib2-algorithm/src/main/java/net/imglib2/algorithm/linalg/matrix/RealCompositeSquareMatrix.java:44: error: reference not found
[ERROR] *         Square {@link RealMatrix} that reads data from {@link Composite}
[ERROR] ^
[ERROR] /home/travis/build/imglib/imglib2-algorithm/src/main/java/net/imglib2/algorithm/linalg/matrix/RealCompositeSymmetricMatrix.java:44: error: reference not found
[ERROR] *         Symmetric {@link RealMatrix} that reads data from {@link Composite}

I guess, RealMatrix is left over from before ojAlgo?
(Just fix and push on master)

[hanslovsky]

Yes, that was from before ojAlgo, I will fix it.
I tried to ensure that it builds running mvn clean build before creating the final PR but that was not enough, apparently. What maven command would I need to run to replicate the behavior on travis? Probably a question for @ctrueden

[tpietzsch]

mvn javadoc:javadoc