Added further test cases and found more problems.
Initial addition of numpy halffloat files.

No modifications made at this point. These files were copied from git
https://github.com/numpy/numpy
on 2013-07-21.
Integrated Numpy half float code successfully.

The numpy code is preferable to the half code for several reasons.
Firstly, it is part of a respected and maintained project, so likely
to be well tested. It is also much easier to read and follow.
Numpy is licensed under a BSD license, so there should be no issue
with including it here.
Removed half code from cellperformance-snippets.