[BUG] Positive formal_charge isn't converted to SDF charge field correctly

[lohedges]

Round-trip read/write of SDF files is failing due to incorrect handling for positive formal charge values on write. According to this resouce we should have:

Formal charge	SDF value
-3	7
-2	6
-1	5
0	0
1	3
2	2
3	1

The code is handled here

sire/corelib/src/libs/SireIO/sdf.cpp

Lines 953 to 976 in ecfef72

	if (atom.hasProperty(map["formal_charge"]))
	{
	int charge = int(atom.property<SireUnits::Dimension::Charge>(map["formal_charge"]).value());
	if (charge >= 4 or charge < -3)
	{
	sdfmol.addProperty("CHG", i + 1, QString::number(charge));
	charge = 0;
	}
	else if (charge < 0)
	{
	switch (charge)
	{
	case -1:
	charge = 5;
	break;
	case -2:
	charge = 6;
	break;
	case -3:
	charge = 7;
	break;
	}
	}

This has no conditional for positive formal charge, so it just takes the formal charge value, i.e. 1 would be written as 1, not 3. I'm also not sure what one does about the additional "doublet radical" option, which should have a value of 4?