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: $ ./apply-filter.py -f GROUND_JOHNSON_B \ spectra/vega/VegaA06-012s06crop4orderData.dat The script is calibrating magnitude to Vega system. The string after -f argument has to match the filter string defined in PyPHOT library.