Added a view exposure_by_country

[micheles]

This is needed by the Aristotle project #9227. We can use the taxonomy mapping corresponding to the country with more assets. Here is what happens for the event based risk demo (Nepal):

$ oq show exposure_by_country
| country | num_assets |
|---------+------------|
| 119     | 24         |
| IND     | 928        |
| NPL     | 8_111      |

NB: also added an utility SiteCollection.split_by_gh3 that will be useful in the future.