Selective/partial parsing of message fragments

[khaeru]

(Copied from the former doc/roadmap.rst.)

This is already done by test_parse_elem():

sdmx/sdmx/tests/reader/test_reader_xml.py

Lines 131 to 137 in 8cd101c

	def test_parse_elem(elem, expected):
	"""Test individual XML elements.
	This method allows unit-level testing of specific XML elements appearing in
	SDMX-ML messages. Add elements by extending the list passed to the
	parametrize() decorator.
	"""

…but could be advertised as a feature in the API. This requires clearly delimiting which elements can be read in this way; some may only be readable with context from the complete message.