Add capacity to inherit DSD from dataflow

[eblondel]

Useful if we could inherit DSD document from a dataflow request, as pointed out in #91.

Since a SDMDataFlows object can handle multiple dataflows, this feature will allows, by specifying dsd = TRUE in readSDMX to associate the DSD to each dataflow, (although primarily intented to be used for a single dataflow). The code to access the DSD is then not the same as for data:

For a SDMXData object, we call the dsd with:

dsd <- slot(sdmx, "dsd")

For a SDMXDataFlows object, we will call the dsd with:

dsd <- slot(slot(sdmx, "dataflows")[[1]], "dsd")