Update default gross to net generation conversion factors

[grgmiller]

According to page 16 of the EIA's Electric Power Monthly Technical Notes, whenever a generator only reports gross generation, they use the following conversion factors to calculate net generation:

Instead of defaulting to setting net generation equal to gross generation when we have no other information, we should use these as default factors.

However, it is unclear whether the prime mover factors and environmental equipment factors are used in combination (i.e. a combined cycle steam plant with flue gas desulfurization would use (0.97 x 0.97 = 0.94)

[grgmiller]

As part of #273, we started using these default prime-mover based factors as the backstop gross to net generation ratio if no other methods worked. However, there are several ways this still needs to be improved:

• Add the impact of environmental equipment to the GTN ratio
• Double check that all prime movers are covered, and identify default gtn factors for those that are not.