bugfix - to force the use of SPM for lead field interpolation

git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8677 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0
enhancement - added option to give an HPI fif file (cfg.hpifile) as input argument


git-svn-id: svn+ssh://svn.fcdonders.nl/home/svnroot/fieldtrip/trunk@8678 0cf7c7f0-3615-4144-b4e6-68da3bce3cd0