Get units correct for species

[luciansmith]

In Antimony, we assume that all species are concentrations, but in SBML, it needs to be flagged with 'substance Units'

So, the Antimony:

species S1
S1 has moles

translates to

<?xml version="1.0" encoding="UTF-8"?>
<!-- Created by libAntimony version v3.0.0 with libSBML version 5.20.4. -->
<sbml xmlns="http://www.sbml.org/sbml/level3/version2/core" level="3" version="2">
  <model metaid="__main" id="__main">
    <listOfUnitDefinitions>
      <unitDefinition id="mole_litre">
        <listOfUnits>
          <unit kind="mole" exponent="1" scale="0" multiplier="1"/>
          <unit kind="litre" exponent="1" scale="0" multiplier="1"/>
        </listOfUnits>
      </unitDefinition>
    </listOfUnitDefinitions>
    <listOfCompartments>
      <compartment sboTerm="SBO:0000410" id="default_compartment" spatialDimensions="3" size="1" constant="true"/>
    </listOfCompartments>
    <listOfSpecies>
      <species id="S1" compartment="default_compartment" substanceUnits="mole_litre" hasOnlySubstanceUnits="false" boundaryCondition="false" constant="false"/>
    </listOfSpecies>
  </model>
</sbml>

which is wrong!