Matches: add getter to retrieve the number of matches between two images

Change-Id: I920994c5fc268fd068d615e15eb64e7e82f225b4
Tracker::Track: fix opencv::assert if not enough features are available

it seems that opencv flann matcher returns invalid indices if
not enough matches are available. In any case if we have less than
4 correspondences the 2d homography cannot be computed.

Change-Id: I31c53af003daaa7758ba984f10b99e677eaf63c4
nvr/CMakeLists: do not link against header files

Change-Id: Ibc6b0cc8303c2ddbab6a0abd8a019e6366f9ebb3
multiview: update some methods to use opencv types

EssentialFromFundamental -> getEssentialFromFundamental
MotionFromEssential -> getCameraSolutionsFromEssential
MotionFromEssentialAndCorrespondence -> getCameraMatFromEssential

added getCameraMatFromFundamental

Change-Id: I692bc7287fc2f25c27dc16e5712305afc4d5cd2d
reconstruction: add findCameraMotionTwoViews + unit test

Computes the camera motion up to scale relative to the first
view in homogeneous coordinates assuming same internal camera
parameters.

Change-Id: Ieb7168c266a9261f4bd212112c2b17acc38e9d90