Toggle between 2D / stack spectrum

[lpatiny]

A 2D spectrum is a set of 1D spectra and sometimes we would like to see them as stacked 1D spectra.

We should therefore add a toggle that allows to switch between contour plot and stacked 1D spectra. This should be done 'per 2D spectrum'. It is not a problem if the other 2D spectra disappear when one of them is in this stacked mode.

This would look like:

And we could think about an horizontal shift from one spectrum to another.

The vertical scale would apply on all the 1D spectra because there is no way to select one of them.

We should also think about making those white peaks. This requires to create a svg filled polygon. Not sure how the xyReduce would behave but anyway the 'optimize' option should be set to 'true'.

It would also be reasonable to display max 128 1D spectra (or a defined number of spectra)

Ref: https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Physical_Methods_in_Chemistry_and_Nano_Science_%28Barron%29/04%3A_Chemical_Speciation/4.07%3A_NMR_Spectroscopy