[apa.cls] Add possibility to cite data sets

This is described here http://blog.apastyle.org/apastyle/2013/12/how-to-cite-a-data-set-in-apa-style.html . However, we will use the newest recommondation about how to format doi's here as well. I added also the case, that it is possible to have data with a title contained in a data set with a title as outlined here https://www.zotero.org/support/dev/translators/datasets

Let me know what you think.

apa.csl

@@ -191,6 +191,19 @@
           </else>
         </choose>
       </if>
+      <else-if type="dataset" match="any">
+        <choose>
+          <if match="none" variable="container-title">
+            <group delimiter=" ">
+              <text variable="title" font-style="italic"/>
+              <text value="Data set" prefix="[" suffix="]"/>
+            </group>
+          </if>
+          <else>
+            <text variable="title"/>
+          </else>
+        </choose>
+      </else-if>
       <else-if variable="reviewed-author">
         <choose>
           <if variable="reviewed-title">
@@ -336,7 +349,7 @@
                     <date-part prefix=", " name="month"/>
                   </date>
                 </if>
-                <else-if type="article-journal bill book chapter graphic legal_case legislation motion_picture paper-conference report song" match="none">
+                <else-if type="article-journal bill book chapter graphic legal_case legislation motion_picture paper-conference report song dataset" match="none">
                   <date variable="issued">
                     <date-part prefix=", " name="month"/>
                     <date-part prefix=" " name="day"/>
@@ -363,7 +376,7 @@
   </macro>
   <macro name="issued-sort">
     <choose>
-      <if type="article-journal bill book chapter graphic legal_case legislation motion_picture paper-conference report song" match="none">
+      <if type="article-journal bill book chapter graphic legal_case legislation motion_picture paper-conference report song dataset" match="none">
         <date variable="issued">
           <date-part name="year"/>
           <date-part name="month"/>
@@ -515,6 +528,12 @@
       <if type="article article-journal article-magazine article-newspaper" match="any">
         <text variable="container-title" font-style="italic" text-case="title"/>
       </if>
+      <else-if type="dataset" match="any">
+        <group delimiter=" ">
+          <text variable="container-title" font-style="italic"/>
+          <text value="Data set" prefix="[" suffix="]"/>
+        </group>
+      </else-if>
       <else-if type="bill legal_case legislation" match="none">
         <text variable="container-title" font-style="italic"/>
       </else-if>