to dynamically set the number of threads (and amount of memory) for
certain rules (create_power_network and estimate_wind_fields) we use a
dynamically generated CSV file with the number of targets in each
country

this file must be present for rules to allocate resources

use the checkpoint lookup in an input function to guarantee its presence

previously, we had removed the checkpoint lookup from the
`threads_for_country` function as per the docs, but this does not seem
to work (snakemake error on comparison of thread count integer and
'TBDString')

maybe this whole scaling resources is too much complexity, but trying to
run a global analysis without it is also inefficient at the
aforementioned rules...