fairland: Tweak rural elevations for simplicity

fairland: Tweak rural elevations for simplicity

elevate_land() computes the sequence of elevations 97, 97 - delta,
... in fixed point with a scaling factor of 100.  Switch to
floating-point, because it's simpler.  Elevations (and thus resources)
change slightly due to reduced rounding errors.

Note that we map elevations less than 1 to 1 both before and after the
patch.  Odd.  Turns out this mitigates a bug: mountain placement can
place fewer mountains than it should, and when that happens,
elevations go below 1 here.  The next commit will fix this.

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