Add support for images with double precision pixel values

[dmehring]

When I attempt to load CASA images with double precision pixel values, I get a pop up window with the message "Error loading file: Cannot load image data. Check log." Given that CASA images support several data types for pixel values, it would be great if CARTA could at least support double precision.

The immediate issue is that there is a limitation with the python-casacore image writing code in that it defaults to writing images with double precision pixel values. It becomes impossible to write images with single precision pixel values once the size of the pixel array exceeds the available system memory. So, although I can have large double precision value images, I cannot view them, at least with CARTA or the casaviewer. The latter has the same issue, and likely won't be changed to support this, as it is deprecated. casacore/python-casacore#257
Thanks