Mandatory options describing what the data is a source of and
when it should be used:
    fieldname={STRING}  -- name of output field calculated from this source
    output_stagger={U V M HH VV} -- the staggering of the output field
    dest_fieldtype={continuous categorical}  -- type of field calculated from src
    priority={INTEGER}  -- priority level of this source (larger number =
                           higher priority)


Options describing other aspects of the source:
    path={STRING}  -- where to find the data
    fill_missing={REAL}  -- value assigned to gridpoints that have no data
    halt_on_missing={yes no}  -- halt if missing values found in output field
    z_dim_name={STRING}  -- for 3-d output fields, name of z dimension
    masked={land water} -- ??? this is field should be masked by land/water


Option to indicate which field the landmask should be computed from; should be 
specified for exactly one fieldname:
    landmask_water={INTEGER}  -- compute landmask based on value of water
    landmask_land={INTEGER}  -- compute landmask based on value of land


For dominant category-type fields:
    dominant_category={STRING}  -- name of dominant category to compute from src
    dominant_only={STRING}  -- only compute a dominant category from src

For derivatives (only for continuous fields):
    df_dx={STRING} -- calculate df_dx for this field, and name the result STRING
    df_dy={STRING} -- calculate df_dy for this field, and name the result STRING

Other options:
    scale_factor={REAL}  -- scale final field by this scalar
    interp_option={sixteen_pt four_pt nearest_neighbor average}  -- interpolation type
    smooth_option={1-2-1 smth-desmth}  -- smoothing type, if specified
    smooth_passes={INTEGER}  -- number of smoothing passes


Mandatory options describing the projection of the source:
These options are used in the `index' file for each source, NOT in the 
input.table
    fieldtype={continuous categorical}  -- type of this source data
    proj={lambert polar mercator regular_ll}  -- projection of source
    dx={REAL}
    dy={REAL}
    missing_value={REAL}  -- value of missing data
    signed={yes no}  -- whether signed in 2's complement format
    known_x={REAL}
    known_y={REAL}
    known_lat={REAL}
    known_lon={REAL}
    truelat1={REAL}
    truelat2={REAL}
    stdlon={REAL}
    units={STRING}
    description={STRING}
    wordsize={INTEGER}  -- how many bytes per value 
    tile_x={INTEGER}  -- number of points in x direction per tile
    tile_y={INTEGER}  -- number of points in y direction per tile
    tile_z={INTEGER}  -- number of points in z direction per tile
    tile_z_start={INTEGER}  -- alternate to tile_z; starting z index
    tile_z_end={INTEGER}  -- alternate to tile_z; ending z index
    tile_bdr={INTEGER}  -- number of halo points around each tile
    category_min={INTEGER}  -- for categorical data, min category in the source
    category_max={INTEGER}  -- for categorical data, max category in the source
