One additional energysource type for accumulators.

[Kingdud]

edit: Realized primary-output would not have the desired effect. It would cause the accumulator to constantly be empty.

tert-in-sec-out: Collects energy as a tertiary source, identical to accumulators in the base game today, but expends energy when a power shortage exists, in the form of a secondary-output source.

Reason for request:

1. Nuclear power is an excellent base-load source. This energy source type would allow a sufficiently large accumulator (such as in the Enhanced Energy Grid Storage mod) to buffer the output of nuclear power and smooth out factory spikes. Mods would need to set their energy generators to 'primary-output' to see utility from this proposed buffer type.
2. It could also be used as a teaching tool, since this would be how real-world energy grids function.
3. Many people currently use steam-buffered nuclear setups in vanilla factorio (if not forever, then at least at one point in time) to achieve this exact desired outcome. This removes the need to use storage tanks full of 500c steam as a workaround for accumulators.