Implement ILU and ILDL preconditioners for scalar matrix entries

This merge request shifts to a new implementation strategy: There are methods toVector and toMatrix now that interpret scalars as FieldVector or FieldMatrix objects, respectively. That allows to get rid of the Hybrid::ifElse pattern, which is legible but longer.

added 1 commit

• f05bd527 - Implement ILU/ILDL preconditioners for scalar matrix entries

Compare with previous version

merged

mentioned in commit ab28696c

I'm sorry for moaning about this change, but as far as I know this cast violates aliasing rules and thus leads to undefined behaviour. While it seems to work for now, I learned the hard way that such violations may lead to very hard to diagnose Heisenbugs.

There's two possible solutions, both involve to return a new value in toVector() and toMatrix() for raw types:

• Create matrix/vector-like proxy classes storing a reference to the raw value and providing the necessary interface.
• Return a new FieldMatrix<B,1,1> / FieldVector<B,1> copying the value.

While the second solution avoids the boilerplate code of the first one, it introduces an artificial copy. However, I guess that the compiler would optimize the potential overhead anyway. But this should be verified.

Yep, I know. I am pretty sure that copying the value is not a problem in optimized code. Unoptimized code may indeed require measurements.