Set output to half precision for RenderVars with half types

[jhodgson]

If we create renderVars with half precision (we currently use the dataType and then override with the format) we need to add HALF at the end of the output string for that AOV in read_options.cpp.

The usual case is that the format is set to color3h for the RGB case, but there may be other scenarios such as as color4h (RGBA), half3 (RGB), half4 (RGBA), half2 (VECTOR2) and half (FLOAT) to support.

[jhodgson]

aov_setup_manual_add_RGBA_from_subnet_v3.usd_rop1.usda.zip

[jhodgson]

Example file with half float AOVs