Generate transactions from PeriodicTransaction within a DateSpan

Note that new transactions require txnTieKnot post-processing.

>>> _ptgen "monthly from 2017/1 to 2017/4"
2017/01/01
    ; recur: monthly from 2017/1 to 2017/4
    a           $1.00

2017/02/01
    ; recur: monthly from 2017/1 to 2017/4
    a           $1.00

2017/03/01
    ; recur: monthly from 2017/1 to 2017/4
    a           $1.00

>>> _ptgen "monthly from 2017/1 to 2017/5"
2017/01/01
    ; recur: monthly from 2017/1 to 2017/5
    a           $1.00

2017/02/01
    ; recur: monthly from 2017/1 to 2017/5
    a           $1.00

2017/03/01
    ; recur: monthly from 2017/1 to 2017/5
    a           $1.00

2017/04/01
    ; recur: monthly from 2017/1 to 2017/5
    a           $1.00

>>> _ptgen "every 2nd day of month from 2017/02 to 2017/04"
2017/01/02
    ; recur: every 2nd day of month from 2017/02 to 2017/04
    a           $1.00

2017/02/02
    ; recur: every 2nd day of month from 2017/02 to 2017/04
    a           $1.00

2017/03/02
    ; recur: every 2nd day of month from 2017/02 to 2017/04
    a           $1.00

>>> _ptgen "every 30th day of month from 2017/1 to 2017/5"
2016/12/30
    ; recur: every 30th day of month from 2017/1 to 2017/5
    a           $1.00

2017/01/30
    ; recur: every 30th day of month from 2017/1 to 2017/5
    a           $1.00

2017/02/28
    ; recur: every 30th day of month from 2017/1 to 2017/5
    a           $1.00

2017/03/30
    ; recur: every 30th day of month from 2017/1 to 2017/5
    a           $1.00

2017/04/30
    ; recur: every 30th day of month from 2017/1 to 2017/5
    a           $1.00

>>> _ptgen "every 2nd Thursday of month from 2017/1 to 2017/4"
2016/12/08
    ; recur: every 2nd Thursday of month from 2017/1 to 2017/4
    a           $1.00

2017/01/12
    ; recur: every 2nd Thursday of month from 2017/1 to 2017/4
    a           $1.00

2017/02/09
    ; recur: every 2nd Thursday of month from 2017/1 to 2017/4
    a           $1.00

2017/03/09
    ; recur: every 2nd Thursday of month from 2017/1 to 2017/4
    a           $1.00

>>> _ptgen "every nov 29th from 2017 to 2019"
2016/11/29
    ; recur: every nov 29th from 2017 to 2019
    a           $1.00

2017/11/29
    ; recur: every nov 29th from 2017 to 2019
    a           $1.00

2018/11/29
    ; recur: every nov 29th from 2017 to 2019
    a           $1.00

>>> _ptgen "2017/1"
2017/01/01
    ; recur: 2017/1
    a           $1.00

>>> _ptgen ""
*** Exception: failed to parse...
...

>>> _ptgen "weekly from 2017"
*** Exception: Unable to generate transactions according to "weekly from 2017" because 2017-01-01 is not a first day of the week

>>> _ptgen "monthly from 2017/5/4"
*** Exception: Unable to generate transactions according to "monthly from 2017/5/4" because 2017-05-04 is not a first day of the month        

>>> _ptgen "every quarter from 2017/1/2"
*** Exception: Unable to generate transactions according to "every quarter from 2017/1/2" because 2017-01-02 is not a first day of the quarter        

>>> _ptgen "yearly from 2017/1/14"
*** Exception: Unable to generate transactions according to "yearly from 2017/1/14" because 2017-01-14 is not a first day of the year        

>>> let reportperiod="daily from 2018/01/03" in let (i,s) = parsePeriodExpr' nulldate reportperiod in runPeriodicTransaction (nullperiodictransaction{ptperiodexpr=reportperiod, ptspan=s, ptinterval=i, ptpostings=["a" `post` usd 1]}) (DateSpan (Just $ parsedate "2018-01-01") (Just $ parsedate "2018-01-03"))
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Check that this date span begins at a boundary of this interval, or return an explanatory error message including the provided period expression (from which the span and interval are derived).