Add indicators intended for normalization (issue #24)

The indicator values are stored as fixed precision
decimal numbers, with up to 11 digits in the integer part
(compared to a maximum of 8 digits seen in the current data)
and up to 11 digits in the fractional part (compared to a
maximum of 10 digits seen in the current data).

I also added 3-letter country codes needed to reference countries
from indicator records, which fixes issue #8:
https://github.com/medea-project/ipcc-database/issues/8