weak namespace and include guards

Currently the OpenEXR Half Float implementation uses less expressive include guard _HALF_H_; makeing a slight modification ie: OPENEXR_HALF_H would allow better integration with other distantly related software packages such as H5CPP a general persistence library for modern C++.

#ifndef _HALF_H_
#define _HALF_H_
...
#endif

And a possible more descriptive, safer alternative:

#ifndef OPENEXR_HALF_H
#define OPENEXR_HALF_H
...
#endif

The suggested change would allow automatic detection of the Float16 header inclusion, providing a better experience to some of your users.

In addition the the include guards, the class half { ... } appears without namespace enclosure; would it be possible to place the class (or the better, all of them) within a namespace? example:

namespace oexr {
....
}

Background: In this HDF5 mailing list post Dr Werner Benger suggested to add OpenEXR/Float16 support to HDF5 C++ or H5CPP. Currently this support has been added with explicit configuration option -DWITH_OPENEXR_HALF

best wishes: steven