Bug: Convert trend to log unit when it differs from the unit of the most recent log

When creating a new monthly log, the code which fills in the trend carry-forward from the last trend value in the most recent existing log failed to convert the trend value from the log unit of that log to the log unit of the new one. This resulted in wild variances if a user changed the log unit and then entered data in a new log. I added unit conversion for this case, which was already handled correctly for the case of complete recalculation of trend carry-forwards (and hence can be used to correct any existing problems due to this bug). (Reported by Eric Carr.)