move uv calculation from casa to python

[Casey Law]

casa measures is not thread safe. uvw calculation crashes when multithreaded.
Probably should just make it Python anyway.

Potential inspiration:

• https://github.com/telegraphic/interfits/blob/master/interfits/lib/coords.py
• https://github.com/telegraphic/interfits/blob/0ee46e94b84d405c8381772be05b42e0b9c41158/interfits/ledafits.py