Fix azimuth encoding

[sfinkens]

Azimuth values are in [-180, 180] since #35, so set offset to 0.0 in order to prevent integer overflow. Relative azimuth difference is in [0, 180] and therefor not affected.

Example: With offset=180 and scale=0.01, a value of azi=-180 would be encoded to -36000, which is beyond the range of int16 (-32768). With offset=0 the maximum encoded values are +/-18000.

[mraspaud]

LGTM

[sfinkens]

For the record: The encoding was fine, because there is actually no offset applied to the angles:

    # Apply scaling & offset
    bt3 -= 273.15
    bt4 -= 273.15
    bt5 -= 273.15
    for array in [bt3, bt4, bt5, ref1, ref2, ref3, sun_zen, sat_zen, sun_azi,
                  sat_azi, rel_azi]:
        array *= 100.0
    for array in [lats, lons]:
        array *= 1000.0

The offset hdf5 attribute (180) was just wrong.