Entire UNRST file is loaded into memory when only headers are asked for

[berland]

In this code snippet:

resdata/python/resdata/resfile/rd_file.py

Lines 404 to 412 in 4f0df4d

	def headers(self):
	"""
	Will return a list of the headers of all the keywords.
	"""
	header_list = []
	for index in range(self.size):
	kw = self[index]
	header_list.append(kw.header)
	return header_list

the entire contents of the UNRST file is loaded into memory.

On sufficiently big UNRST files, this will lead to an out-of-memory situation.

This bug is responsible for equinor/subscript#755

[berland]

This can be seen by stepping through the code and checking that the kw object contains data for each cell in the model.

Seemingly garbage collection is insufficient to purge the loaded data for each loop invocation.

[berland]

res2df has been ported to use resfo instead of this code, thus working around this limitation.