Remove rounding of reference region base year cost

Rounding is causing issues when the base year cost is below 0.5 (such is the case for some cooling technologies). These values are rounded down to 0, which then causes NAs for cost projections.

message_ix_models/tools/costs/regional_differentiation.py

@@ -225,7 +225,7 @@ def subset_module_map(raw_map):
             "reg_diff_source.notnull() or base_year_reference_region_cost.notnull()"
         )
         .rename(columns={"base_year_reference_region_cost": "base_cost"})
-        .assign(base_year_reference_region_cost=lambda x: x.base_cost.round())
+        .assign(base_year_reference_region_cost=lambda x: x.base_cost)
         .drop(columns={"base_cost"})
     )