[feature] CV_16F support in HDF5

[braindevices]

HDF5 v2.0.0 finally introduced Float16 support. So we should take the advantage to allow save/load CV_16F in opencv.

Right now we have to cast it to CV_16S then save as int16 in HDF5, at the load side we have to do the cast again.

#4074 allow us to directly save CV_16F data as it is with proper metadata