Add issort flag for Eigenvalue calculation

[ksugar]

This PR adds an option to not perform sorting in the eigenvalue calculation.

In some cases, the sorting operations are unexpected.

[tinevez]

Hello.

Just to sure: they are not sorted by default?

Can you add more to the doc:

• and state in what order they will be sorted,
• on their value or on their absolute value?
• and stress that the vectors will be sorted as well, by the eigenvalues also.

[ksugar]

Hello.

Just to sure: they are not sorted by default?

They are sorted by default, which is the same behavior as before (I tried to keep the backward compatibility).

I did not modify the original sorting algorithm, which works as follows:

• eigenvalues (non-absolute) are sorted in ascending order accompanied by its corresponding eigenvectors

I will explicitly write it in the doc comments.