Query population usign city boundaries to avoid loading all country data

[Claudio9701]

Currrent behaviour:

pop_search = up.download.search_hdx_dataset(country)
pop_country = up.download.get_hdx_dataset(pop_search, pop_index) # This takes too much time

Requested feature:

pop_search = up.download.search_hdx_dataset(country)
pop_country = up.download.get_hdx_dataset(pop_search, pop_index, mask=city_limits) # This is expected to be faster

Where city_limits represent either the city boundaries as a polygon or the city total bounds as a bounding box.