Fixed bug where changing sensor altitudes to satellite or sea level fails after custom altitude set
Made AOT argument to custom altitude function optional.

The AOT argument can be set to -1 (the default value within Py6S)
which tells the AOT to be interpolated from a standard 2km exponential
profile. This isn't specified in the 6S manual, but is specified in the
comments of the 6S code.
Added test for the changing altitudes bug.