[eigen] skipXxx / computeXxx parameters in Eigenvalues module

[Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>]

Hello,

The SelfAdjointEigenSolver class allows for the computation of only the 
eigenvalues and not the eigenvectors. This allows a non-trivial time 
saving. My plan is to extend this functionality to the other eigensolvers 
(this is not much work).

However, there is an API issue that I'd like some opinions about. The 
compute() function in the SelfAdjointEigenSolver class has a bool argument 
computeEigenvectors; the eigenvectors are computed iff it is true. The 
compute() function in the Tridiagonalization class has a bool argument 
extractQ; the matrix Q (which is analogous to the eigenvectors) is 
computed iff it is true. However, the compute() function in the 
ComplexSchur class has a bool argument skipU; the matrix U is computed iff 
it is *false*.

I think this inconsistency is potentially confusing for the users. On the 
other hand, changing it may break existing programs. So what to do?

Cheers,
Jitse