#938 DateAxis.previousStandardDate with standard date as input

[Roderick Locke]

DateAxis.previousStandardDate doesn't return the previous "standard" date when you give it a date that is itself a standard date.

For instance, with the axis timezone set to UTC and a DateTickUnit of
new DateTickUnit(DateTickUnit.MILLISECOND, 250)
the input
new Date(0)
will yield a date with a timestamp of -1, rather than the timestamp of -250 that I expect.

The attached patch is for version 1.0.9, but the issue exists in the trunk as well. Not all of the tests pass; some of the sanity checks for DateTickUnit.MONTH aren't working. There seems to be an incompatibility between GregorianCalendar.add and Month.previous (which doesn't accept a timezone and so can't be used in DateAxis.previousStandardDate).

[Roderick Locke]

Patch for DateAxis and DateAxisTests (which doesn't pass)