Add Convergent Source

[JLBegin]

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[marcandrevigneault]

Great idea to add this type of source.
One thing I don't like off the bat is that we have DivergentSource and ConvergentSource which both use a different method of telling the angle of spread. DivergentSource directly gets the divergence full fan angle and ConvergentSource uses focal. I think both should use the same metric OR allow both of them to be defined by either focal or angle.