Feature/measurement api clang warning

[Kerstin-Keller]

This PR intends to fix the clang warnings in imeasurement.h/omeasurement.h.

For doing this, another template parameter was introduced, but hidden from the user.

In general, the interface has not changed except for two points:

• A different file needs to be included, see
• The function signature of the Get function for getting a channel from a measurement now has two arguments instead of just one. Happy for any idea on how to solve this!
• The message providers are now in the namespace eCAL::message::protobuf, eCAL::message::string. Does it make sense to have it like this, or rather the other way round? (eCAL::protobuf::message, eCAL::string::message)....

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"