Potential ramp rate issue in first snapshot when new generators have a build year not equal to the first investment period (multi-horizon only)

[PeterKlein11]

Checklist

• [ x] I am using the current master branch or the latest release. Please indicate:

Describe the Bug

In single year models the first snapshot is ignored for ramp rates. In multi-horizon models, this seems to only happen for generators where the build_year is equal to the first investment period. For later build years, the ramping condition is included in the first snapshot the generator is active. Generally this isn't a problem, but in cases where a p_min_pu is specified in that hour and a lowish ramp rate this can lead to infeasibilities as the generator is expected to ramp up to the p_min_pu faster than the ramp rate allows.

Possible fix is to remove the ramping constraint from first snapshot for all new generators at their build_year, or to modify the ramp rate in that snapshot to be 1 to avoid infeasible situations. Alternatively a warning to the user when such conditions are specified.