Add d3_hierarchy subclass

This is to support Unipept API tool and visualization:   https://github.com/unipept/unipept-visualizations
The unipept visualization would only apply to the d3_hierarchy of json datasets.

config/datatypes_conf.xml.sample

@@ -61,6 +61,7 @@
     <datatype extension="bowtie_base_index" type="galaxy.datatypes.ngsindex:BowtieBaseIndex" mimetype="text/html" display_in_upload="false"/>
     <datatype extension="csfasta" type="galaxy.datatypes.sequence:csFasta" display_in_upload="true"/>
     <datatype extension="data" type="galaxy.datatypes.data:Data" mimetype="application/octet-stream" max_optional_metadata_filesize="1048576" />
+    <datatype extension="d3_hierarchy" type="galaxy.datatypes.text:Json" mimetype="application/json" subclass="true" display_in_upload="false"/>
     <datatype extension="data_manager_json" type="galaxy.datatypes.text:Json" mimetype="application/json" subclass="true" display_in_upload="false"/>
     <datatype extension="dbn" type="galaxy.datatypes.sequence:DotBracket" display_in_upload="true" description="Dot-Bracket format is a text-based format for storing both an RNA sequence and its corresponding 2D structure." description_url="https://wiki.galaxyproject.org/Learn/Datatypes#Dbn"/>
     <datatype extension="fai" type="galaxy.datatypes.tabular:Tabular" display_in_upload="true" subclass="true" description="A Fasta Index File is a text file consisting of lines each with five TAB-delimited columns : Name, Length, offset, linebases, Linewidth" description_url="http://www.htslib.org/doc/faidx.html" />