Add recommendations for viscosity and velocity for K17 kernel and move the check to kernel

The viscosity and velocity for which we can use a collision kernel are specific to the kernel variant. Therefore, the checks should be executed by the Kernel class and not by Parameter.

Additional changes:

• in unit tests: remove cout capture with testing::internal. testing::internal is a private API of GoogleTest and therefore should not be used like for non-internal purposes like as was here. Another reason for the removal is that for the unit tests added in this merge, capturing cout with testing::internal did not work with msvc.
• update spdlog
• rename namespace testingVF to testing::vf