[DF] Add proper support for multidimensional arrays

Multidimensional arrays can be read into RDataFrame as unidimensional arrays using RVec. This is unsatisfactory as the shape information contained in the input file is lost.

The ideal solution would not require users to specify the shape by hand as that would allow the same syntax for fixed and variable size arrays (and would not require that users know/remember what size the array should be).