Python: Document (parts of) ExternalArtifact.qll.

I don't think there's any need to document the parts specific to
metrics or defects, as I don't believe these are used anywhere.

Author: tausbn

python/ql/src/external/ExternalArtifact.qll

@@ -1,3 +1,7 @@
+/**
+ * Provides classes for working with external data.
+ */
+
 import python
 
 class ExternalDefect extends @externalDefect {
@@ -27,23 +31,37 @@ class ExternalMetric extends @externalMetric {
     string toString() { result = getQueryPath() + ": " + getLocation() + " - " + getValue() }
 }
 
+/**
+ * An external data item.
+ */
 class ExternalData extends @externalDataElement {
+    /** Gets the path of the file this data was loaded from. */
     string getDataPath() { externalData(this, result, _, _) }
 
+    /** Gets the path fo the file this data was loaded from, with its
+     * extension replaced by `.ql`.
+     */
     string getQueryPath() { result = getDataPath().regexpReplaceAll("\\.[^.]*$", ".ql") }
 
+    /** Gets the number of fields in this data item. */
     int getNumFields() { result = 1 + max(int i | externalData(this, _, i, _) | i) }
 
+    /** Gets the value of the field at position `index` of this data item. */
     string getField(int index) { externalData(this, _, index, result) }
 
+    /** Gets the integer value of the field at position `index` of this data item. */
     int getFieldAsInt(int index) { result = getField(index).toInt() }
 
+    /** Gets the floating-point value of the field at position `index` of this data item. */
     float getFieldAsFloat(int index) { result = getField(index).toFloat() }
 
+    /** Gets the value of the field at position `index` of this data item, interpreted as a date. */
     date getFieldAsDate(int index) { result = getField(index).toDate() }
 
+    /** Gets a textual representation of this data item. */
     string toString() { result = getQueryPath() + ": " + buildTupleString(0) }
 
+    /** Gets a textual representation of this data item, starting with the field at position `start`. */
     private string buildTupleString(int start) {
         start = getNumFields() - 1 and result = getField(start)
         or