Fix subscriptions for GRADPCA

[ypnos]

The GRADPCA dependency on GRAD is broken in the distribution views. If the latter is not computed due to other means, the former just fails. Instead, it should trigger computation of the latter. GRADPCA is disabled for now in master until this is fixed. See 4977772

[ypnos]

The problem is that imagemodel, or somebody else(?) needs to subscribe on GRAD whenever GRADPCA is computed. Controller has a default subscription on IMG for the same reason.