Widget to fill 2-dimensional array of things

[sverhoeven]

Nested arrays are not rendered very intuitively by rjsf.

In PR #53 a table widget was added. It requires an array of objects where the rows are the array items and the columns are the properties of the object.
It would be nice to have an widget which works on an array of arrays of scalar or object. The rows could be the outer array and the columns the inner array. The cell could be the input field for the scalar or a sub-form for the object.

Something like

	inner 1	inner 2	inner 3	Add column
outer 1	sub-form
outer 2
Add row

Based on #48 (review)

[sverhoeven]

The alascan module in expert catalog has resdic (List of residues to be mutated) property which is such a format.

which produces

[alascan]

resdic_A = [
  12,
  13,
  14,
]

Which is not very friendly due to double plust and minus buttons.