file id of moment images

[kswang1029]

(Not sure if this is frontend or backend issue)
To reproduce the issue:

1. load set_HD163296/HD163296_CO_2_1.fits
2. create a rectangle region at the center and make moment images
3. match the resulting moment images to the spatial reference (aka HD163296_CO_2_1.fits)
4. regenerate moment images from HD163296_CO_2_1.fits and we will see
   

When we re-request a new set of moment calculations, how the file_id of the previous moment image is handled? Do we need "CLOSE_IMAGE" involved?

[markccchiang]

@kswang1029 The file_id of the previous moment image will be closed, i.e. via the CLOSE_IMAGE command. Does this error messages still appear in the backend branch mark/refactor_moment_generator?

[kswang1029]

@markccchiang Tested with mark/refactor_moment_generator, it works well without those error messages. Close this for now.