Avoid using namespace std/cv

From cpp core guidelines (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf6-use-using-namespace-directives-for-transition-for-foundation-libraries-such-as-std-or-within-a-local-scope-only):

Reason using namespace can lead to name clashes, so it should be used sparingly. However, it is not always possible to qualify every name from a namespace in user code (e.g., during transition) and sometimes a namespace is so fundamental and prevalent in a code base, that consistent qualification would be verbose and distracting.

This seem to happen in trackerItem.cpp:

C:\Users\tobias\dev\petrack\src\trackerItem.cpp(37,52): error C2872: 'Tracker': ambiguous symbol [C:\Users\tobias\dev\p
etrack\build\petrack_core.vcxproj]
C:\Users\tobias\dev\petrack\include\trackerItem.h(28,7): message : could be 'Tracker' [C:\Users\tobias\dev\petrack\buil
d\petrack_core.vcxproj]
C:\Users\tobias\dev\petrack\build\vcpkg_installed\x64-windows\include\opencv2/video/tracking.hpp(713,20): message : or
      'cv::Tracker' [C:\Users\tobias\dev\petrack\build\petrack_core.vcxproj]

For avoiding such clashes, it might be better to remove using namespace std/cv;, a explicitly state what to call.