Fix custom age groups
Created by: wonder-sk
The expressions were not correct - processing algorithm's input parameters are not available in expressions automatically neither as fields (columns) nor as variables. One way of fixing (with minimal changes) would be to use string concatenation in python and do something like:
'eval( ... generate_series(' + parameters['Uppersecondarystartingage'] + ', ...)'
But the whole expression is already quite complex, so the fix comes with a simplification to directly build the expressions in Python without resorting to complex QGIS expressions.