Clarify functions which retrieve pubkey sources and balance

get_sources() and get_amount_from_pubkey() return the lists of sources inputs and pubkey’s balance.

Solution 1

• use dataclass or clarify its usage in comments, inside this two f() and in f() around them.
• rename to pubkey_balance second returns argument

Solution 2

• Split f(), one to retrieve sources, one to calculate from those sources, pubkey’s balance.

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#113 (closed): 8) rename value to amount