Clarify the licence of OTB standalone package

As you know OTB is a project under Apache V2 license based on ITK which is also under Apache v2 license. However ITK has some modules (ITKFFT, ITKPDEDeformableRegistration found with a grep of ITK_USE_FFTW) which use FFTW implementation (under GPL v>=2). A clear message indicate in the case of ITKFFT that you must comply with GPL licence.

In a previous discussion during license migration we have define that in the case of ITK is build with FFTW support all the ITK code is under GPLv3 (as for their debian package). Therefore in this case OTB binary package build on it is under GPLv3. Some points:

• it is quite abusive for me: only some parts of the OTB code, in this case, must comply with GPLv3 license: modules which linked with related ITK modules
• binary packages distributed by us must expose a LICENSE file which indicate that in this case the package is composed of components with different licenses. Currently only a LICENSE file related to Apache v2 is exposed in root directory.
• all otb debian sub-packages must expose a GPLv3 license due to the fact that this package is build on a ITK package which marked as GPLv3 (on their side they have only one package)
• I propose that the otb debian master package keep expose an Apache v2 license

Moreover they are another issue with this binary packages because they include now remote modules with different license (AGPL v3, CeCILL v2, GPL v3, Apache v2, CeCILL-B v1). These different licenses are not indicated in the root directory. I think we should adapt our packaging script to put them.

I think we need to clarify this point for the users which use this binary package to build more complex applications. If they use cli, no issue but if they use python wrapping or lib available they need to take care of which app / module they use to define the license of their code.