Fix SAX handler implementations to prevent value truncation

As noted in the commits, value parsing is moved from the ```characters(...)``` method to the ```endElement(...)``` method in the SAX handler for Operetta and ScanR data.  This prevents values from being truncated when multiple calls to ```characters(...)``` are necessary to process a single value.

This should fix http://hudson.openmicroscopy.org.uk/view/Bio-Formats/job/BIOFORMATS-full-repository-develop/