BF: explicitly specify to use bash instead of generic POSIX compliant sh

As reported in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690622 some
scripts have "bashisms" -- features which are present only in bash but not in a
limited, but still POSIX compliant shells such as dash (default on many
systems nowadays to speed up boot etc).

Here is a current list from running bashishsms tool

possible bashism in Scripts/ANTSAverage2DAffine.sh line 22 (let ...):
for x in $LL ; do  PARAM1=` awk -v a=$PARAM1 -v b=$x 'BEGIN{print (a + b)}' ` ;  let NFILES=$NFILES+1  ; done
possible bashism in Scripts/ANTSAverage3DAffine.sh line 29 (let ...):
for x in $LL ; do  PARAM1=` awk -v a=$PARAM1 -v b=$x 'BEGIN{print (a + b)}' ` ;  let NFILES=$NFILES+1  ; done
possible bashism in Scripts/guidedregistration.sh line 4 (unsafe echo with backslash):
echo " USAGE \n  sh command.sh  fixed.nii fixedhipp.nii  moving.nii movinghipp.nii outputname  iterations DIM "
possible bashism in Scripts/landmarkmatch.sh line 4 (unsafe echo with backslash):
echo " USAGE \n  sh $0  fixed.nii fixedhipp.nii  moving.nii movinghipp.nii  ITERATIONS LandmarkWeight "
possible bashism in Scripts/landmarkmatch.sh line 28 (let ...):
let DD=MM-NN
possible bashism in Scripts/landmarkmatch.sh line 37 (let ...):
let DD=MM-NN
possible bashism in Scripts/lohmann.sh line 45 (alternative test command ([[ foo ]] should be [ foo ])):
if [[ ! -s ${OUTPUTNAME}repaired.nii.gz ]] ; then
possible bashism in Scripts/lohmann.sh line 51 (alternative test command ([[ foo ]] should be [ foo ])):
if [[ ! -s $WM ]] ; then
possible bashism in Scripts/lohmann.sh line 57 (alternative test command ([[ foo ]] should be [ foo ])):
if [[ ! -s  ${OUT}max.nii.gz ]] ; then
possible bashism in Scripts/weightedaverage.sh line 3 (unsafe echo with backslash):
echo "Usage: \n sh  weightedaverage.sh  \"Faces*tiff\" \n "

although alternative is to fix the scripts, but IMHO it would be better just to
switch to use bash (present everywhere anyways) for the shell interpreter for
those scripts.