Implementing camera

- [x] Spheres trajectory widget
- [x] setViewerPose, getViewerPose
- [x] setCamera, getCamera
- * In order to simulate the intrinsics of a camera, the projection matrix is directly set in vtkCamera from Camera class
- * vtkCamera itself doesn't allow setting projection matrix, therefore I inverted and premultiplied it to have identity as projection matrix, then I premultiplied this identity matrix with my Camera's projection matrix.
- [x] Camera class implementation
- * Camera class requires window size.
- * Additionally, it requires either field of view, intrinsic matrix or intrinsic parameters
- * Default principal point is set to the middle of the given window size, focal length computed accordingly.
- * If the window size is updated, camera changes its horizontal field of view accordingly (when aspect ratio changes).
- [x] window to world, world to window transformation
- * Used vtk methods