[nodes/bash_regis] Create a function that give the path of the bash directory

[BastienCagna]

[nodes/bash_regis] Create a function that give the path of the bash directory

[davidmeunier79]

Done, the directory bash is now called in all wraps of nodes calling it, e.g.:
`

package_directory = os.path.dirname(os.path.abspath(__file__))
_cmd = 'bash {}/../bash/T1xT2BiasFieldCorrection.sh'.format(
    package_directory)

`