testing GC with sweeptool

## Abstract

For non-coders with some CLI knowledge. Also iOS 13 or higher needed.

We want to test if `sweeptool-cli` and [Gordian Cosigner](https://github.com/BlockchainCommons/GordianCosigner-iOS) app are interoperable.

* First, install Gordian Cosigner (GC) and familiarize with it (try to create/import cosigners, create an account map, a psbt etc.)
* In Gordian Cosigner we can export an account (Bitcoin output descriptor) (*) and import it into `sweeptool-cli` to see if it works.
* For example, working with GC in testnet mode, you can first try to create an account with 2 cosigners created from seed or bip39 words. Then try to send some testnet coins to that account (google for a testnet faucet). Export that account into `sweeptool-cli` and sweep those funds to any address of your liking. The sweeptool-cli will create a PSBT for you which you can import back into GC and sign it with your cosigners. Can the transaction actually be signed and broadcast? Show and document the steps you've taken and PR a document. Report if you run into problems.

/* the app may export a descriptor without a tail (1/* and 0/*) which should be appended manually, make notes

Bonus work: come up with more test scenarios

Installieren Sie Gordian Cosigner auf iOS 13 oder höher und verwenden Sie sweeptool-cli mit einem Testnet-Konto, das als Bitcoin-Output-Descriptor exportiert wurde. Folgen Sie dem Erstellen-, Finanzieren-, Exportieren-, Sweep-, Importieren-, Signieren- und Broadcast-Ablauf des Issues und notieren Sie jeden Descriptor-Anhang, der angefügt werden muss. Dokumentieren Sie die genauen Schritte und Ergebnisse in einem PR, einschließlich Problemen und aller getesteten Bonus-Szenarien.

Vom Indexierungsmodell aus dem Issue-Text verfasst.