Allow replacement of supervision control block reference, closes #79.

[danyill]

Closes #79.

I also don't know if any of this is a good idea.

I've left a few formatting updates from npm run format in here.

[JakobVogelsang]

Dear @danyill. I just had the time to look at your RP. Thank you for pushing the scl-lib.

There are two option that I see.

1. Replace be inherently set when the supervision.iedOrLn is LN. Then we just move the existing supervision to a new logical node. This required the iedOrLn be empty (no Val element with inner text)
2. Remove the supervision before creating a new one. For this could just export the removeSubscriptionSupervision function. Then still all check would apply and you would have some fail savety.

[JakobVogelsang]

There are updates on #79. I would like to close this PR and finalize the refinement on #79 first.