wsclean - delete items before moving

[tjgalvin]

In some circumstances the in memory temporary file system (the ramdisk) is used during wsclean in order to lighten the load on the larger filesystem. This is done through a context manager.

When the scope of the context manager is terminated, files in a watch directory are copied to a final location. It is only once the files have been copied are they examined while cleaning up. This cleaning up should be done while the data are still in memory.

Similarly, if fits cubes are being created from any subband images, these could be created and then moved.

[tjgalvin]

This was done in #192