Add code for integrating spectrum over passband

This preliminary code is based on PyPHOT library.  In order to test
passband integration with an example spectrum, execute it from the
command line as follows:

  $ ./ -f GROUND_JOHNSON_B \

The script is calibrating magnitude to Vega system. The string after -f
argument has to match the filter string defined in PyPHOT library.
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