Added rpath setting for PlaidML backend

Fix crash on MacOS for r0.23

Use libc++ for macOS core python wheel

Minor PlaidML fixes

pydocstyle 4.0 breaks flake8-docstrings.

This came up while debugging memory corruption in the ngraph-tf tests: plaidml_ops_replicate was holding onto
a reference to a temporary that was destroyed while the reference was still live.  Replacing that code with a
call to get_argument() eliminated the temporary and fixed the issue.  And in general, there's no reason to be
allocating temporary vectors when we're pulling out a single argument, so this change replaces the other
sequences of get_arguments().at(), too.

The PrefixReshapeElimination pass doesn't work correctly if the network's operation's input and output shape
compatibility is being validated; an implicit broadcast operation is required to get the shapes right.  This
change moves the implementation back to PlaidML (since it's seeming less generally useful, and PlaidML has an
implicit broadcast operation), and fixes both it and the implicit broadcast pass to only apply when the
downstream operation is an elementwise operation (since PlaidML doesn't support automatic broadcasting for
contractions).

Update setup.py

Improve PlaidML backend option debugging

Decomposition of fused ops in PlaidML backend [r0.23]

Add PlaidML Quantize & Dequantize

Added a missing guard that is breaking the build