* Changed behavior of `readJournalFiles` to be identical to `readJournalFile` for singleton lists
* Balance Assertions have to be simple Amounts
* Add 'isAssignment' and 'assignmentPostings' to Hledger.Data.Posting and Transaction
* Implemented 'balanceTransactionUpdate', a more general version of 'balanceTransaction' that takes an update function
* Fixed test cases.
* Implemented balance assignment ("resetting a balance")
* Add assertions to show function
* updated the comments
* numbering is not needed in journalCheckBalanceAssertions
* remove prices before balance checks
* rename functions
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