FP16 support in serialization

* Allow serialization of fp16 data
* Add package to support integrated half data-type (half_float::half), independent of native float: http://half.sourceforge.net/


* Allow passing of accumulate data-type in serialization

Signed-off-by: James Ward <james.ward@arm.com>
Change-Id: I54357f02e3776d81958228f699ea5044f2014f4b