Allow ordering the data query results by dimensions

[dosse]

Linked to ticket #50, which we use to retrieve only partial data that fit into a size-limited, paginated, online pivot table, the need is for a user-defined order of the retrieved observations/series. Since the tables are pivot tables, the required content (observations) of the first (and subsequent) table pages depend on the table layout (distribution of dimensions on different tables axes, like rows and columns). The table layout thus dictates which slices of observations need to be returned per page. Therefor, the SDMX web services call should be able to accept a user-defined sequence of dimensions, which is used to order the observations before they are returned.

The order within one dimension should take the code order (respectively the hierarchical code order) into account.

[sosna]

Fixed by #185