Constraints on habitat descriptions are omitted

[jocelynpender]

Constraints like "usually" are not parsed currently. This is something that would be desirable in habitat descriptions.

See the following excerpt from V19_742.xml from the FNA.

 <description type="habitat">
    <statement id="habitat_0">
      <text>Relatively dry, usually sandy soils, old fields, pastures, usually disturbed;</text>
      <biological_entity id="hab_o0" name="whole_organism" name_original="" type="structure">
        <character name="habitat" value="Relatively dry sandy soils" />
        <character name="habitat" value="old fields" />
        <character name="habitat" value="pastures" />
        <character name="habitat" value="disturbed ;" />
      </biological_entity>
    </statement>
  </description>