Support for instantiating Half-Typed-grid from stored float or double grids

[konivo]

This is still WIP and it depends on #1730 .

The main motivation for this patch is to be able to load half-typed grids directly from files containing non-half-typed grids in order to keep the process peak memory low. When having large grids or large number of grids this can be important.

The PR does (atm) the conversion according to the following rules:	stored grid value type	desired conversion	returned grid
float	half	half
double	half	half
vec2f	half	vec2h
vec3f	half	vec3h
vec2d	half	vec2h
vec3d	half	vec3h

The above rules can be extended if desirable, and in the future there could be also 'store' counterpart to do conversion into other grid types while storing. That could potentially replace the need for "saveFloatAsHalf" flag.

The core implementation has been already tested with various vdb files, some of them using saveFloatAsHalf, and the approach seems to be working without issues.

The implementation is straightforward and follows these steps:

• User specifies desired conversion when calling File::open or when instantiating Stream
• Desired conversion information then propagates into StreamMetadata
• GridDescriptor::read picks up the conversion from the metadata and from io::ConvertingReaderFactory it gets a reader and the resulting grid valuetype. It then instantiates a grid with this new valuetype instead of using the original value type. If there is no suitable reader for the desired conversion, the method instantiates a grid with the original valuetype.
• The reader returned from io::ConvertingReaderFactory is later also stored in StreamMetadata and used during topology/buffers loading.

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• :white_check_mark: login: konivo / name: Ivo Kondapaneni (c292c28bb86e9490867d4a09c26881d51e99c047)