Merge different sensors within one station

It could be useful to be able to merge different sensors into a single one within a given station. For example, merging TA_THY and TA_ROT into TA so the resulting TA is TA_THY when TA_THY is available and TA_ROT otherwise.

This should be build on the current MERGE InputEditing algorithm, simply using the same source and destination station. For example by making the MERGE argument optional: it would be empty for self-merge and otherwise contain a station ID (as is currently the case).