New option fill_value_subst

[ch-k]

This option can be used in conjunction with GeoImage.fill_value. Any occurrences of
fill_value within the image data will be replaced with fill_value_subst before storing
to image file.

Example trollduction configuration to use this feature:

<file>test.tif
    <format_params>
        <fill_value_subst>1</fill_value_subst>
    </format_params>
</file>

[coveralls]

Coverage increased (+0.3%) to 12.213% when pulling 8dcaa3b on khunger:feature-fill-value-substitution into d95c944 on pytroll:pre-master.