Added ValueType to Variables and update default waterml cvs list

[lsetiawan]

Overview

This PR should complete https://github.com/ODM2/WOFpy/issues/160#issuecomment-326142439. Though not the complete mapping for ValueType, some ODM2 ActionType CV might still get filtered.

Demo

<variableCode default="true" vocabulary="mysqlodm2timeseries" variableID="4">USU4</variableCode>
<variableName>Turbidity</variableName>
<valueType>Observation</valueType>
<dataType>Average</dataType>
<generalCategory>Water Quality</generalCategory>
<sampleMedium>Surface water</sampleMedium>

[emiliom]

woo-hoo! Reviewing now.

[emiliom]

Looks good