Market splitting

This fixes an unknown error in the null market's order splitting decision. I discovered it by running minimal_input/source_3_sink_1.xml and found that the sink was requesting 3 kg, each source was offering 1 kg. The logic went as follows:

compare 1 kg to 3 kg: match
compare 1 kg to 2 kg: match
compare 1 kg to 1 kg: no match

Accordingly, I changed the logic from a strict > to >=.