Move using namespace units to namespace mantle_api

- using the namespace "units" globally creates a name clash of
units::inverse with the glm library's glm::inverse, which is used within
the glm library. glm lib doesn't compile anymore then
- moved "using namespace units;" to mantle_api namespace for now

Signed-off-by: Andreas Rauschert <andreas.rb.rauschert@bmw.de>

MantleAPI/include/MantleAPI/Common/spline.h

@@ -24,8 +24,6 @@
 
 #include <tuple>
 
-using namespace units;
-
 namespace units
 {
 UNIT_ADD(jerk,
@@ -58,6 +56,8 @@ UNIT_ADD_CATEGORY_TRAIT(jerk_acceleration)
 
 namespace mantle_api
 {
+using namespace units;
+
 template <typename T>
 struct SplineSegment
 {