Improve HexBlock.getFlowArea() calculation

[john-science]

Right now, the HexBlock.getFlowArea() calculation only calculates the area based on Flags.COOLANT, but @clstocking reports it could also be dependent on Flags.INTERDUCTCOOLANT.

From the current calc:

armi/armi/reactor/blocks.py

Line 2594 in 6fcc481

return self.getComponent(Flags.COOLANT, exact=True).getArea()

To something more like:

area = self.getComponent(Flags.COOLANT, exact=True).getArea()
for c in self.getComponents(Flags.INTERDUCTCOOLANT, exact=True):
    area += c.getArea()

return area