New tool: add_aux_band

Adds another band to the input file with incidence angles or doppler
or whatever the user requests on the command line.  For example:
  add_aux_band incidence_angle in out

So far I have implemented incidence angle, look angle, slant range,
doppler, time, latitude, and longitude.

This is for JPL, who needs to be able to add incidence angles as a
second band before geocoding in order to get those values without
having to perform terrain correction.
Build new tool add_aux_band by default.
Merge pull request #129 from asfadmin/add_aux_band

New tool for Don and JPL - add_aux_band