Update mods2geoblacklight.xsl

mods has more than one type of relatedItem. the attribute 'host' is used to indicate Collection.
I added this attribute to the selection parameter.
Also changed xsl:for-each to xsl:if since there should only be 1 parent collection for each layer
Merge pull request #23 from kimdurante/patch-3

Update mods2geoblacklight.xsl