Interpolating DEM data.

Interpolating DEM data.

- DEM data can now be interpolated using ione of 3 methods:
  o no_interpolation: Take the nearest south-west data.
  o simple_interpolation: Interpolated using a simple method.
  o best_interpolation: Interpolated using Shepard method.

- "Apply DEM data" for a track now uses "best_interpolation".

- Elevation-distance graph now uses "best_interpolation".

- Elevation information displayed on status bar now uses most suitable
  interpolation methods according to the current zoom level.

Not much testing has been made to justify performance/accuracy trade-off
of the interpolating methods.

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>