Refactor the duplicated Faust::power_iteration() codes into one function for...

Refactor the duplicated Faust::power_iteration() codes into one function for all Faust::LinearOperator hierarchy of classes (CPU).

- Before the code was duplicated for Transform and MatDense (and not existing for MatSparse). Deleted the duplicate in faust_transform_algebra.
- Added the prototye LinearOperator::multiply(Faust::Vect) and defined it in child classes MatSparse/Dense and Transform. With needed utility template functions (operator * overloads) in faust_linear_algebra.
- Minor change elsewhere (faust_int type in power_iteration replaced by int).