Some C++ header files contain using namespace foo

[levy]

According to C++ conventions this is considered bad practice. The namespace will be also used in the C++ files where the header is included even if the file is not the implementation but a simple user.

[ZoltanBojthe]

currently used:
using namespace omnetpp;
using namespace inet::units::values;
using namespace units::constants;
using namespace ::osg;

all other "using namespace" removed from headers