McXtrace/McStas 3.0 : Mask needs the masking variable to be atomic.

[ebknudsen]

For 2nd gen cogen, Mask uses the masking variable to extract the pixel value of the mask at the point of intersection, for use with the virtual operation. This is obviously no longer possible. In the McXtrace-3.0 branch this is wrongly fixed using a USERVAR which cannot be declared in-component (yet?). For McStas-3.0 masking is simply allowed to be set on the component struct (old behaviour), which leads to a race condition.