Change or rename hermitian_decomposition()
I'm opening this as a separate issue, because it is design-related, not code-related.
In my opinion, the name "hermitian_decomposition" is misleading for the operation that the Space and Field methods currently perform. Of any data array there is exactly one hermitian decomposition, and it therefore cannot depend on the value of additional flags like "preserve_gaussian_variance". I suggest to move the necessary manipulations for variance preservation into a separate routine. Alternatively the method name could be changed, but I can't think of anything convincing.