Add option of linear interpolation

Add option of linear interpolation for spectroscopy images
alignment. The interpolation is done between the first and
the last image.
This option has been proposed and developed by ICMAB users
(Miquel Garriga and others).
New option -f=2 to linearly align spectroscopies

Miquel Garriga added to the project authors list.
Modifications to adapt SW for Mistral beamline and test new
option -f=2 with a normalized spectroscopy.
Update license to GPL

Update license to GPL and add a copy of the license in the file:
COPYING.