heads up: removing convert_pair_to_loop() from gemmi

[wojdyr]

Hi,
I replaced a gemmi function that is used here:

freesasa/src/cif.cc

Lines 713 to 716 in 51a996d

	if (!table.loop_item) {
	// Table is a pair so turning it into a loop
	table.convert_pair_to_loop();
	}

with ensure_loop(). Sorry about this. This is the only github project that uses this function (from what I found in github search), so I didn't leave the old name for backward compatibility.
These 4 lines can be replaced with table.ensure_loop();, or perhaps table.erase() can be used instead.