Add tool for ingesting variables from dbgap

[cmungall]

E.g.

https://ftp.ncbi.nlm.nih.gov/dbgap/studies/phs000228/phs000228.v4.p1/pheno_variable_summaries/phs000228.v4.pht002156.v1.EMMES_HMP_DSU.data_dict.xml

e.g

  <variable id="phv00158592.v1">
    <name>DSUDIET</name>
    <description>Diet</description>
    <type>Integer, encoded value</type>
    <logical_min>1</logical_min>
    <logical_max>5</logical_max>
    <value code="01">01-Meat/fish/poultry at least three days per week</value>
    <value code="02">02-Meat/fish/poultry at least one day but not more than two days per week</value>
    <value code="03">03-Eggs/cheese/other dairy products, but no meat/fish/poultry</value>
    <value code="04">04-No animal products at all (no meat/fish/poultry, no eggs/cheese/other dairy products)</value>
    <value code="05">05-No animal products and at least 70% of fruits and vegetables are eaten raw</value>
  </variable>

• name -> name
• description -> description
• type -> range
• max/min -> max/min
• values -> permissible values in enum