dasymmetric interpolation

[mountainMath]

extend tongen_estimate to allow for effective dasymmetric interpolation.

Possible applications:

• use GHSL data to better estimate where people live, GHS-BUILT-S_GLOBE_R2023A should be useful for that. Download could focus on just Canada to provide fully-automated dasymmetric interpolation for all censuses in the satellite area (so back to 1975).
• Use custom data available at specific regions to pinpoint where people live, e.g. Metro Vancouver Land Use data that distinguishes residential from other land and the land use intensity.
• Anther option is to use geo-located address registries like the NAR to pinpoint dwelling units.

This will likely greatly improve accuracy when using tongfen_estimate