Change default thresholds used when calculating plume maps

In CO2Leakage we calculate plume maps from the aggregate maps MAX_SGAS and MAX_AMFG. We should use the same defaults as elsewhere for SGAS and AMFG/XMF2, see #137.

Is there any way to make it easier for the user to use the same thresholds for all scripts (if the user want to change some threshold)?

NB: We might decide to calculate and store the plume maps outside CO2Leakage. In that case, we need to make sure the defaults are correct when we calculate the maps in the aggregate script. See #216 .