Codec Framework and Delayed Loading Removal

[danrbailey]

This introduces the new codec framework and completely removes delayed-loading / out-of-core into the master branch.

All of the components included in this PR have been reviewed independently:

#2155
#2164
#2166
#2179
#2180
#2184
#2185
#2197
#2196
#2230

These changes have been tested extensively at ILM.

For completeness, here are the new additions to the changelog on this branch from aggregating all the pending changes:

OpenVDB:
    Highlights:
    - Completely removed support for out-of-core / delayed loading across OpenVDB
      core, including removal of the OPENVDB_USE_DELAYED_LOADING build option,
      DelayedLoadMetadata, and TempFile. This was done to improve performance,
      reduce codebase complexity and to entirely eliminate the optional Boost
      dependency.
    New features:
    - Introduced new openvdb::codecs subsystem and CodecRegistry for stream
      encoding and decoding.
    - Added support for implicit codec conversion fallback (e.g. float to half,
      scalar to mask) and ReadDiagnostics.
    API changes:
    - Extracted Point Data I/O function overloads from PointDataGrid.h into new
      openvdb/points/PointDataIO.h header.
    - GridDescriptor::read() has been deprecated, use GridDescriptor::readHeader()
      followed by GridDescriptor::readStreamPos() instead.
    - Added optional trailing WriteOptions parameter to io::File::write() and
      io::Stream::write(). No changes in behavior.
    - Added optional trailing ReadOptions parameter to io::File::getGrids() and
      io::File::readGrid(). No changes in behavior.
    - TreeBase::readTopology() and TreeBase::writeTopology() are now pure virtual.
      Derived classes of TreeBase now need to read and write int32_t(1) to remain
      backwards-compatible.
    Improvements:
    - Large refactor of I/O classes, many private and protected member functions have
      been modified. No change in behavior for io::File and io::Stream.
    - Remove direct testing of Tree I/O methods in favor of indirect testing using
      the Grid I/O.
    Houdini:
    - Add new Vulkan viewport support for VDB Points primitives and remove support
      for delayed loading.