add fns to calculate exposure and vulnerability from gridded pop and human development index data

[nlebovits]

Data are now pulled from:

• GHSL's 100m gridded pop density data (https://developers.google.com/earth-engine/datasets/catalog/JRC_GHSL_P2023A_GHS_POP#bands)
• Human development index at 100m resolution (https://gee-community-catalog.org/projects/gridded_gdp_hdi/)

Flood probability, which is 0 to 1, is multiplied by minmax scaled pop data for the bounding box of the country in question, which gives an exposure score from 0 to 1.

Exposure is then multiplied by z-score normalized HDI data to give a vulnerability score.

Both exposure and vulnerability are written as geotiffs to the relevant /outputs cloud bucket.

Note: this latter score can/should be updated to also be minmax scaled (previously I was working from GDP, so normalizing made more sense).

[olivegardener]

are you using both HDI and GDP to calculate population vulnerability. or just HDI?