Cross-tracking opened PSI issues

[luke-kiernan]

Issues opened in PSI that I've fixed here:

• PSI #1531: added dt multiplier in market_bid.jl code path of VOM cost
• PSI #1530: swapped current and previous terms in ramp down limit

Check off as they resolve: if it turns out that any of these aren't actually bugs, then I'll need to re-introduce that functionality here.

[jd-lara]

Issue #1530 maybe a lack of commenting on the code what the expr down does.

[luke-kiernan]

Just in case issue 1530 is correct as-it-was: it's the Union{PowerAboveMinimumVariable, ActivePowerVariable} implementation of add_linear_ramp_constraints!.

[jd-lara]

yeah there are different implementation for expression than for variables