polar volumes with existing mistnet parameters

[adokter]

In certain cases polar volumes may be encountered that already have mistnet related parameters added to the volume (e.g. WEATHER, BIOLOGY, etc).

In these cases, currently vol2bird will not update those existing parameters, and will also give an incorrect warning message:

vol2bird/lib/librender.c

Lines 849 to 852 in eb41d8d

	if(PolarScan_hasParameter(scan, "WEATHER")){
	fprintf(stderr, "Warning: scan used multiple times as MistNet input, ignoring segmentation %i/%i\n", iScan+1, MISTNET_N_ELEV);
	continue;
	}

Need a mechanism that overwrites the old parameter with a warning message