This license is chosen for several reasons:

- Although the Xilinx portions are unlicensed, they use BSD 3-clause
  elsewhere in the company under similar circumstances
  (https://github.com/Xilinx/HLx_Examples/blob/master/LICENSE.md)

- Pybind11 is also licensed under 3-clause BSD, so we aren't adding any
  additional restrictions.

- Anything more restrictive is unlikely to be productive anyway, since
  this code is unlikely to be distributed to end-use customers.

PLEASE NOTE that the license has a comment at the top indicating the
unlicensed Xilinx portions. It seems like a no-brainer that Xilinx is OK
with code re-use (it's a thin shim around their libraries) but IANAL.

Comments welcome.