do not hard-wire column names in annotations (#785)

* enforce column name uniqueness for obs and var

* parameterize the column name containing obs and var user-readable names

* use the new annotation index value from schema

* update f/e unit tests

* PR review suggestions

* lint

client/src/components/continuous/continuous.js

@@ -66,7 +66,8 @@ class Continuous extends React.Component {
           ? _.map(obsAnnotations.colIndex.keys(), key => {
               const isColorField =
                 key.includes("color") || key.includes("Color");
-              if (key === "name" || isColorField) return null;
+              if (key === schema.annotations.obs.index || isColorField)
+                return null;
 
               const summary = obsAnnotations.col(key).summarize();
               const nonFiniteExtent =