yoda: add an option to switch off export of multidimensional objects

[GraemeWatt]

Add an option like no_multi_dim that switches off export of Scatter3D objects (and possibly higher dimensions, see #47) and instead writes multiple Scatter2D objects, one for each independent variable. This was the behaviour of the YODA exporter in the old HepData site and it still makes sense for some tables.

Since a Scatter3D object would still be preferred for covariance matrices, the code could check the description for 'covariance', 'correlation' or 'matrix' and still write a Scatter3D object in these cases even if the no_multi_dim option is set. See https://gitlab.com/hepcedar/rivet/-/issues/318 .