Fixes NoneType error in wfdb.io.wrann when using label_store instead of symbol #260

[briangow]

symbol needs to be defined in calc_core_bytes so the annotations can be converted into bytes for writing. I added this to define symbol when label_store is being used:

 # Calculate the symbol field if necessary
        if 'symbol' not in present_label_fields:
            self.convert_label_attribute(source_field=present_label_fields[0],
                                         target_field='symbol')

[Lucas-Mc]

Syntax looks good and consistent and it works!