Enable SDMX-ML SAX (Simple API for XML) parsing method

[eblondel]

When SDMX-ML documents become large, ending up with 2 copies of the data in memory at once, i.e the tree and the R datastructure (resulting from as.data.frame in rsdmx), could lead to memory issues.
In this case, event-driven or SAX (Simple API for XML) style parser could be investigated, not as replacement but as complementary approach to read SDMX-ML files. With the XML package, event-driven parsing is possible using xmlEventParser.

Under this activity, an enhancement of readSDMX should allow reading directly from URL (for SDMX web-services that do not require an explicit user-agent). Combined with SAX approach, it intends to provide a powerful SDMX reader that parse SDMX data from the web and minimizes the R in-memory not storing the SDMX-ML XML content, ideal for processing R web-services using rsdmx.

This activity is listed as priority enhancement for which rsdmx seeks for fundings. See https://github.com/opensdmx/rsdmx/wiki#8-fundings
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