[bug] fix popsift static release

[simogasp]

Since alicevision/popsift/pull/71, popsift is now always releasing the resources with the destructor.

In our implementation of ImageDescriber_SIFT_popSIFT popsift is a static member of the class to avoid multiple instances of the Cuda run-time.
Since static object are destroyed last, it happens in alicevision_extractFeatures that the popsift destructor tries to release resource when the Cuda runtime is already shut down.
This causes an error on exit from the program:

~/dev/alicevision/sandbox/popsift/src/popsift/s_image.cu:259
    Could not destroy texture object: driver shutting down

In order to fix the problem, a static counter in the class ImageDescriber_SIFT_popSIFT has been added to count how many instances of the class there are and when the last one is destroyed, also the static popsift is destroyed (the old good "the last one shut the door" principle :-) )

This is a mitigating solution in that it is efficient in managing multiple instances of ImageDescriber_SIFT_popSIFT running at the same time, but obviously it has the drawback that it introduces overheads for allocating/deallocating cuda stuff when several ImageDescriber_SIFT_popSIFT instances are created and destroyed in sequence.

In alicevision ImageDescriber_SIFT_popSIFT at the moment is only used with a single instance so there are no issues.

Incidentally, it cleans up a little the constructors/destructors of the other feature classes with missing explicit, default and override. Sorry, the OCD kicked in. :-)