Added concepts for seasonal, conditional ratings

[getorymckeag]

Contributes to (doesn't close) #14, #129.

Documents concepts, decision records.

[getorymckeag]

@catkins-miso My plan here is to revisit these obviously and fill the consequences out with actual examples and spec pointers after you have that done. Unsure if it's helpful to merge this now, or wait until you're done with the work for #14

[catkins-miso]

@getorymckeag I think your plan is to update this with examples following the approved seasonal ratings spec changes. I also think those examples would include day/night so we can close #123