allow masking conditions to be specified on predictions

An earlier implementation of the masking conditions assumed that the
conditions on streamflow would only be on the observations, but this is
not always the case. For example, reliability scores cannot be done on
the observed streamflow and need to be performed on the predicted
streamflow. So this is now possible as the condition syntax is changed
and now *q_obs*/*q_prd_median*/*q_prd_mean* in place of *q*.