Preventing self-masks from diverging

Masks can be used in various ways in MAKEMAP. For instance, the dimmconfig_bright_extended.lis file specifies:

ast.zero_snr = 5

which causes the areas of the map that have a signal-to-noise ratio of less than 5 to be masked on each iteration. This is a dynamic mask, in that a new and potentially different mask is generated and used on each iteration. However, the added flexibility introduced by allowing such a mask to change on each iteration can result in eventual divergence of the iterative  map-making algorithm. To prevent this, a set of new configuration parameters has been added that result in the mask being frozen after a specified number of iterations. These parameters are AST.ZERO_FREEZE, COM.ZERO_FREEZE and FLT.ZERO_FREEZE. Each of these parameters may be set to a positive integer value, indicating that he corresponding mask should be frozen after that many iterations. They all default to zero, which reproduces previous behaviour (i.e. the mask is never frozen).