Adjust saving preference

[yuema137]

The general logic of the saving strategy could be summarized as:

1. The plugins that involve heavy computation should be saved at least once (there could be duplications in subsequent plugins)
2. The plugins that could be easily calculated from parents should not be saved

The veto_regions and event_pattern_fit fall into the first categories. As they are not computed again for other child plugins, we should always save them during their own computations.

[coveralls]

Changes unknown
when pulling 89fd16d on adjust_saving_preference
into ** on master**.