Fix building number calculation for district with no information

For the calculation of number_building for districts with no information, we first identify the 10 closest population districts. Then we have divided the average of the number of buildings from these 10 districts to the population of one district. ( I am not able to put the next lines of code, typed in my changes in the comment)

I think that's wrong, and we have to take an average of the proportion of number of buildings per person i.e. :

\fyi @ds what do you think ?