Applies a mask to a region of an NDF

#### Description:

This routine masks out a region of an NDF by setting pixels to the bad value, or to a specified constant value. The region to be masked is specified within a text file (see Parameter REGION) that should contain a description of the region in the form of an "AST Region". This is the system used by the AST library for describing regions (see SUN/211 or SUN/210). Such text files can, for instance, be created using the Starlink ATOOLS package (a high-level interface to the facilities of the AST library).

#### Parameters:

##### CONST = LITERAL (Given)
The constant numerical value to assign to the region, or the string "Bad". ["Bad"]
The name of the input NDF.
If a TRUE value is supplied, the constant value is assigned to the inside of the region. Otherwise, it is assigned to the outside. [TRUE]
##### OUT = NDF (Write)
The name of the output NDF.
This copies input NDF a1060 to the output NDF a1060_sky, setting pixels bad if they are contained within the Region specified in text file "galaxies.txt".