Add LisVector::getAbsMaximum().
Add LisVector default constructor creating zero size vector.
Replace internal LisMatrix::LIS_VECTOR _diag with LisVector.

Corrects updating of _diag in addValue().
Add LisMatrix::getDiagonal().
Replace redundant code in LisMatrix::getMaxDiagCoeff().

This also extents the function for the case of already assembled matrix.