Include generator_operating_date in the core_eia860m__changelog_generators table

[arengel]

Is your feature request related to a problem? Please describe.
In some of our models we use monthly 860 data to define the set of generators since it has all the columns we need and reflects the best available data on planned retirements and additions. We'd like to be able to use core_eia860m__changelog_generators for this purpose but it is missing the generator_operating_date which makes this inconvenient.

Describe the solution you'd like
I'd like to have the generator_operating_date as reported in 860m included in the core_eia860m__changelog_generators table.

Describe alternatives you've considered
We could merge on that single column from _out_eia__yearly_generators but that's less convenient and it's not clear if those dates are actually the ones, or the same as the ones reported in 860m.

[e-belfer]

@arengel This should now be fixed in the nightly builds.