bugfix - the 'pos' field should not be removed from the list of fields to be processed (selected/averaged). in test_ft_selectdata this caused avgoverpos to fail. The problem was introduced in https://code.google.com/p/fieldtrip/source/detail?r=10075


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@10087 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0