Include "mesh_term" in JSON schema

[buddha314]

The clinicaltrials.gov API includes the fields "MeSH term"

`

<xs:complexType name="browse_struct">
xs:sequence
<xs:element name="mesh_term" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
`

These provide a rudimentary summary of the trial. Please include them in the JSON