CASA3K: update to casacore 3.3 API

CASA3K: update to casacore 3.3 API

code/CMakeLists.txt: old hand-rolled Boost detection broke; use CMake…

…'s scheme

code/CMakeLists.txt: old hand-rolled Boost detection broke; use CMake…

…'s scheme

gcwrap: fix Python modules on macOS/py37

The problem is that now, if a module links with libpython, you will get a
segfault. This seems to be the case not only for the various CASA Python
extension modules themselves but also for CASA's `libtools.dylib`, which they
link to. I've copied the linker flags manually since I can't figure out how to
figure them out on-the-fly in any reasonable way.

gcwrap/CMakeLists.txt: try to avoid linking libtools with libpython

CASA3K: non-mechanical fixes for Python 3

CASA3K: non-mechanical fixes for Python 3

Merge remote-tracking branch 'nrao/release/5.1.2' into casa3k-5.1

gcwrap/tools/StdCasa/conversions_python.cc: map strings to lists

Previously CASA would map C++ strings to numpy arrays with byte-sequence
dtypes. This is OK in Python 2, but in Python 3 these values don't stringify
nicely: `str(value) = "b'BLAH'"`. So, special-case string arrays to be
rendered into standard lists rather than Numpy arrays, since we already have
the machinery to Unicode-ify them correctly in either Python 2 or 3.