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.
Merge pull request #59 from neurodebian/master

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