Deflation structure preservation

[jarlebring]

There is room for improvement for our implementation of deflation. From the "main idea" (my SIAM CSE presentation):

From the main idea one can see that a deflated SPMF can be viewed as a SPMF with extra terms. Therefore, if we apply a deflation to an SPMF (or AbstractSPMF), I think deflation should return an AbstractSPMF. Also, when we carry out deflation on a deflated problem, for efficiency reasons we can update the deflation rather than to create recursive structure.