Added a function tower_churn_rate for calculating the churn in tower between weeks
Added a function tower_churn_rate for calculating the churn in tower between weeks

yvesalexandre/bandicoot,ulfaslak/bandicoot,ulfaslak/bandicoot,econandrew/bandicoot,econandrew/bandicoot,yvesalexandre/bandicoot,ulfaslak/bandicoot,yvesalexandre/bandicoot,econandrew/bandicoot