Make soil type 'C' the default for missing values

[rajadain]

Number to soil type mapping is available here: https://github.com/WikiWatershed/model-my-watershed/blob/develop/src/mmw/apps/modeling/geoprocessing.py#L38-L52

# The soil rasters contain the numbers 1 through 7 (the keys of this
# dictionary).  The values of this dictionary are length-two arrays
# containing two strings.  The first member of each array is the name
# used for the corresponding soil-type in the TR-55 code.  The second
# member of each array is a human-readable description of that
# soil-type.
SOIL_MAPPING = {
    1: ['a', 'A - High Infiltration'],
    2: ['b', 'B - Moderate Infiltration'],
    3: ['c', 'C - Slow Infiltration'],
    4: ['d', 'D - Very Slow Infiltration'],
    5: ['ad', 'A/D - High/Very Slow Infiltration'],
    6: ['bd', 'B/D - Medium/Very Slow Infiltration'],
    7: ['cd', 'C/D - Medium/Very Slow Infiltration']
}

Connects https://github.com/WikiWatershed/model-my-watershed/issues/1015

[mmcfarland]

+1 looks good.