IVGCVSW-3221 Refactor Mean ref workload and tests

IVGCVSW-3221 Refactor Mean ref workload and tests

 * Renamed RefMeanFloat32Workload and RefMeanUint8Workload
   to RefMeanWorkload, updated references to reflect this
   change.
 * Refactored RefFloorWorkload to use Decoders/Encoders,
   to support the use of multiple data types.
 * Deleted reference Unit8 Mean tests as they were
   duplicates of the Float32 tests. Refactored these tests
   to support multiple data types and updated references.
 * Adjusted the values used in the tests' input tensors so
   that they are more like floating point numbers
   e.g. change 1.0f to 1.5f.
 * Replace size_t with unsigned int in Mean ref workload,
   for better compatibility with the Encoder/Decoder,
   removed some unnecessary casts after this.
 * Added ValidateTensorDataTypesMatch() function to
   WorkloadData.cpp, added CreateIncorrectDimensionsErrorMsg
   function to RefLayerSupport.cpp.
 * Added passing and failing tests for ref IsMeanSupported.

Signed-off-by: James Conroy <james.conroy@arm.com>
Change-Id: Id3d44463d1385255c727a497d4026d21a49e7eb2