* continue the decomposition until a pivot is exactly zero;
don't try to compute the rank in the decomposition itself.
* Instead, methods such as rank() use a new internal parameter
called 'threshold' to determine which pivots are to be
considered nonzero.
* The threshold is by default determined by defaultThreshold()
but the user can override that by calling useThreshold(value).
* In solve/kernel/image, don't assume that the diagonal of U
is sorted in decreasing order, because that's only approximately
true. Additional work was needed to extract the right pivots.
* switched lu/qr tests to be using createRandomMatrixOfRank
* removed unused methods doSomeRankPreservingOperations
* removed NOTE about doSomeRankPreservingOperations
few bits left of the comma and for floating-point types will never return zero.
This replaces the custom functions in test/main.h, so one does not anymore need
to think about that when writing tests.
* bugfix in Dot unroller
* added special random generator for the unit tests and reduced the tolerance threshold by an order of magnitude
this fixes issues with sum.cpp but other tests still failed sometimes, this have to be carefully checked...
