Better Handing of HD Public Keys in Multisig TXs

[mflaxman]

• Add smart understanding of xpubs in PSBTs, so that output descriptors are not required to sign when these are properly supplied. Update multiwallet with this UX.
• Modularize input and output description in PSBT class so that this can be applied to other script types (especially p2sh) in the future
• (re)generalize from_hd_pub and from_hd_priv path handling (don't falsely specify root_path when xpub use-cases are base_path
• Helper method to create a PSBT with hd_pubs and other metadata
• More test coverage
• Minor cleanup of previous codebase

[mflaxman]

This fixes #42