Changes since kapuahi

• If multiple types of mask are requested for any of the AST, FLT and COM models (e.g. ZERO_LOWHITS and ZERO_MASK both specified), they will now be combined into a single mask. Previously, only one of the requested masks would be used.
• Up to three external masks can now be specified using ADAM parameters REF, MASK2 and MASK3. The ZERO_MASK configuration parameter can be set for the AST, FLT or COM model, to indicate which ADAM parameter to use.