ie_precision: added FloatTrait to fix VS2015 client compilation

[mshabunin]

Related PR #5654

Details:

Our VS2015 build has failed due to some unsupported template constructs:

c:\deployment_tools\inference_engine\include\ie_precision.hpp(440): error C2891: 'T': cannot take the address of a template parameter (compiling source file dnn.cpp)  
c:\deployment_tools\inference_engine\include\ie_precision.hpp(439): note: see declaration of 'T' (compiling source file dnn.cpp)

I understand that VS2015 is quite old and is not officially supported by OpenVINO, but since the fix is not too complex, I hope it won't break anything and can be accepted.

This patch is more naive version of the original code and I hope someone can propose better and more elegant solution in review comments. Other idea was to extend PrecisionTrait with something like is_float field or method, seemingly more extensive this change can simplify the code as a whole by omitting is_floating function completly.

cc @apertovs @ilya-lavrenov @alalek

[mshabunin]

Updated to second proposed solution.