Support Matrix return types

[eddelbuettel]

This PR implements something discussed following #222: for (dense) arrays with row and colum indices, we can optionally return a Matrix object (in addition to the existing default of a list of vectors, and the existing data.frame option.

To illustrate, consider the quickstart_dense example:

> library(tiledb)
> arr <- tiledb_array("quickstart_dense", as.matrix=TRUE)
> arr[]
     [,1] [,2] [,3] [,4]
[1,]    1    2    3    4
[2,]    5    6    7    8
[3,]    9   10   11   12
[4,]   13   14   15   16
> 

In case more than one attribute is returned from a query, a list of matrices is returned.