ch07-03 accumulate across countries

Max: Avoid loosing previous country population while stepping to another country.

Hi there, please merge this fix for the Geography module.

With fix:
```
iex(112)> Geography.total_population Geography.make_geo_list("geography2.csv"), "Spanish" 
13455613
```
Without fix:
```
iex(113)> c "geography2.ex"                                                              geography2.ex:1: warning: redefining module City
geography2.ex:5: warning: redefining module Country
geography2.ex:9: warning: redefining module Geography
[Geography, Country, City]
iex(114)> Geography.total_population Geography.make_geo_list("geography2.csv"), "Spanish" 
4877481
```
Manual check:
```
iex(116)> 3255944 + 1621537 + 7737002 + 841130
13455613
```