oxford asl 6 : fix_bolus should be a flag or dropdown rather than string param

[MontrealSergiy]

presently oxford asl fix_bolus parameter is string, yet there is no need for it. One either fixes bolus or allow for variable bolus. The label should be also 'Fix bolus duration', 'Bolus duration' label is already used for --bolus parameter.

see the source of oxford_asl

# fix the bolus duration?
if [ -z $fixbolus ]; then
    basil_options=$basil_options"--infertau "
    echo "Varaiable bolus duration" >> $log
else
    echo "Fixed bolus duration" >> $log
fi