Bug: Setting log weight unit in XML import

Import of an XML database set the weight unit of the log only from the {\tt log-unit} in the preferences, and did not allow the {\tt weight-unit} in an individual monthly log (which might be different) to override the default. I added code to set the unit for a given month from its own {\tt weight-unit}. Note that logs created in the process of importing CSV or XML data are always created using the user's current log unit setting; the log unit in the data imported is used to convert weight values (if necessary) from the unit in the imported log.