fairland: Compute resources by interpolating tables

The functions mapping elevation to resources are piecewise linear.
Replace them by linear interpolation on data points defined in tables.
Tables are easier to tweak for deities than code.

The data points are a bit awkward because the functions they replace
are.  The next commit will tweak them to address that.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>