add utils for is_time_variant

Co-authored-by: jd-lara <[email protected]>

src/utils/powersystems_utils.jl

@@ -338,6 +338,10 @@ function _get_piecewise_incrementalcurve_per_system_unit(
     return PSY.PiecewiseStepData(x_coords_normalized, y_coords_normalized)
 end
 
+function is_time_variant(cost_function::PSY.FuelCurve{PSY.PiecewisePointCurve})
+    return isa(PSY.get_fuel_cost(cost_function), IS.TimeSeriesKey)
+end
+
 function get_deterministic_time_series_type(sys::PSY.System)
     time_series_types = IS.get_time_series_counts_by_type(sys.data)
     existing_types = Set(d["type"] for d in time_series_types)