The nullifier algorithm was wrongly assuming that it would received an
ordered sequence of transactions. When combined with the cancelation
search only looking forward, this could cause the algorithm to not only
not throw the `MoreThanOneMatchError`, but also attempt to rewrite a
previously matches transaction.
The fix is applied both in ordering every sequence of transactions, but
also to confirm that a transactions had not previously been found. This
should now be impossible given the correct order, but its there to
prevent future misshaps.
For the specific broken sequence, look at test
TestTransform.test_nullifier_inplace_unordered.
and allow for multiple null rules on the Nullifier.
Also create an empty list on the Nullifier constructor to simplify the
logic afterwards, in the case no "null" rule exist.
The `CategorySelector` was possibly added to be incremented with other
attributes. However, since none other that the selector enum is used at
the moment, it is only adding unnecessary cluter.
The category selector value is moved to the parent
`TransactionCategory`.
