Possible shared memory issue in fast-rechunk.py

[piyanatk]

The fast-rechunk.py script seems to have some bugs that caused prevent data from being copied from the shared memory to the UVData, yielding zeros or artifacts in the data array. Rerunning the script seems to resolve the issues, which point to some bugs related to shared memory.

The figure below shows the amplitude of the rechunked visibilities as a function of frequencies from the problematic run. Between LST 8 to 12 from ~80 MHz onward were completely empty while there appear to be digital-like artifacts at LST 20