Changes in Version 1.2

  o better handling of ti objects with some NA values.  Thanks to Chuck
    Mikolajczak for pointing out the problem.

  o the observed attribute of a tis can now be one of "beginning", "ending",
    "summed", "averaged", "annualized", "high", or "low".  The last two are new,
    and denote series for which only the high or low value over a period is
    recorded. The convert() function can work with the new attributes.
