Move dac cost reduction assumptions to module folder

Currently the DAC technologies' cost reduction and reduction scenarios assumptions are added to the `energy` module's CSV files. With #255, there is now the functionality to add module-specific cost assumptions. So I am moving the DAC technologies' cost reduction categories and reduction scenarios to its own module directory.

message_ix_models/data/costs/dac/cost_reduction.csv

@@ -0,0 +1,7 @@
+# Cost reduction in 2100,,,,,
+# ,,,,,
+# Units: %  ,,,,,
+message_technology,very_low,low,medium,high,very_high
+dac_lt,0.1,0.3,0.5,0.7,0.9
+dac_hte,0.1,0.3,0.5,0.7,0.9
+dac_htg,0.1,0.3,0.5,0.7,0.9

message_ix_models/data/costs/dac/scenarios_reduction.csv

@@ -0,0 +1,4 @@
+message_technology,SSP1,SSP2,SSP3,SSP4,SSP5,LED
+dac_lt,low,medium,medium,high,high,low
+dac_hte,low,medium,medium,high,high,low
+dac_htg,low,medium,medium,high,high,low

message_ix_models/data/costs/energy/cost_reduction.csv

@@ -96,7 +96,4 @@ wind_res_hist_2020,0.15,0.3,0.53,0.53,0.65,0.75
 wind_res1,0.15,0.3,0.53,0.53,0.65,0.75
 wind_res2,0.15,0.3,0.53,0.53,0.65,0.75
 wind_res3,0.15,0.3,0.53,0.53,0.65,0.75
-wind_res4,0.15,0.3,0.53,0.53,0.65,0.75
-dac_lt,0.1,0.3,0.5,0.7,0.9
-dac_hte,0.1,0.3,0.5,0.7,0.9
-dac_htg,0.1,0.3,0.5,0.7,0.9
+wind_res4,0.15,0.3,0.53,0.53,0.65,0.75

message_ix_models/data/costs/energy/scenarios_reduction.csv

@@ -89,6 +89,3 @@ wind_res1,high,medium,low,high,medium,very_high
 wind_res2,high,medium,low,high,medium,very_high
 wind_res3,high,medium,low,high,medium,very_high
 wind_res4,high,medium,low,high,medium,very_high
-dac_lt,low,medium,medium,high,high,low
-dac_hte,low,medium,medium,high,high,low
-dac_htg,low,medium,medium,high,high,low