Export only used cylinders to divelogs.de

This attempts to export only used cylinders to divelogs.de. The
detection is based only on o2 percentage as per our test dives. However,
the new cylinder id would be better, but that does not exist on old
dives.

Fixes #757

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

xslt/divelogs-export.xslt

@@ -109,6 +109,17 @@
 
     <ADDITIONALTANKS>
       <xsl:for-each select="cylinder[position() != $cylinder]">
+        <xsl:variable name="gas">
+          <xsl:choose>
+            <xsl:when test="@o2 != ''">
+              <xsl:value-of select="substring-before(@o2, '.')"/>
+            </xsl:when>
+            <xsl:otherwise>
+              <xsl:value-of select="'21'"/>
+            </xsl:otherwise>
+          </xsl:choose>
+        </xsl:variable>
+        <xsl:if test="following-sibling::divecomputer/event[@name='gaschange' and @value=$gas]">
           <xsl:variable name="cur_cyl">
             <xsl:value-of select="position()"/>
           </xsl:variable>
@@ -147,6 +158,7 @@
               <xsl:value-of select="substring-before(@he, '%')"/>
             </HEPCT>
           </TANK>
+        </xsl:if>
       </xsl:for-each>
     </ADDITIONALTANKS>