[Pixel Type] add long long as type of dataImage

[mathildemerle]

One of our user had a "long long" pixel type mha image, which can't be read in medInria. It has been solved casting it in an other tool (sitk) to img = sitk.Cast(img,sitk.sitkUInt8).

However, it could be fine to add the possibility to read long long images in medInria.

I just tested to add itkDataImageLongLong3.h and itkDataImageLongLong4.h in src/plugins/legacy/itkDataImage/data and adapt everything but i have a problem with ITK. Indeed, medInria 3.4 is on ITK 5.0, and medInria 4 on ITK 5.1.1. However, until ITK 5.2 the bridge between ITK and VTK does not support long long pixel type.

So, we could add the reading and writing of long long images on medInria 4 with the ITK updating PR : #1132, but not before.