Fix statement line entry date year based on the statement date

[nieldw]

After a bit more than a year I finally managed to re-implement this according to our discussion on PR #83.

This implementation infers the correct year for the entry date of a statement line based on the shortest interval between the entry date's month-day and the statement date. For MT940 the statement date is taken to be the date of the closing balance (:62F: or :62M); for MT942 the statement date is the date-time indicator (:13D:).

After reading the whole statement page, the statement lines are post-processed to adjust the entry dates, because in the case of MT940 the statement date is only available after reading all the statement lines.

Caveat: The entry date is an optional field, but if it is absent the StatementLine constructor assumes it to be the same as the value date. In this case the post-processor may choose an incorrect year.

Closes #83

[nieldw]

@qoomon , please advise if this PR corresponds with the conventions, package structure, etc. that you wish to maintain in this project. Thanks