add histograms on splom diagonals

Several other implementations of splom allow for a different plot type on the diagonal - which makes sense because the diagonal subplots are always just diagonal lines of points. What we really want is a simple view of the distribution of that one dimension in each diagonal subplot. For my current purposes the best view would be a histogram, and I suspect implementing only that would be a well-received improvement, but eventually box and violin could also be nice additions.

We anticipated this feature with a comment in #2555 and with a comment in the attributes for splom:

https://github.com/plotly/plotly.js/blob/f0fef6fe8c1c37786dcc9789bb60576073d2d0b7/src/traces/splom/attributes.js#L143-L158

That comment I think is right, trace.diagonal.type sets the trace type for all of the diagonal subplots, then we add more attributes into trace.diagonal that mirror the attributes available to histogram (and the other trace types if we add those). For histograms, trace.diagonal.marker.color should be inherited from trace.marker.color (and maybe similar with marker.line.width and marker.line.width?)

In addition, each dimension should be able to override everything about its own diagonal subplot, so trace.dimensions[i].diagonal should accept all the same options as trace.diagonal.

Inizia dal commento e dagli attributi indicati in src/traces/splom/attributes.js, quindi segui come vengono configurati i subplot diagonali di splom. Definisci il supporto per gli istogrammi tramite trace.diagonal, incluso il colore del marker ereditato, e le sostituzioni per dimensione tramite trace.dimensions[i].diagonal. Il lavoro è completo quando i subplot diagonali possono mostrare istogrammi con quei percorsi di configurazione funzionanti.