Improve docstring — add reference to Broeg et al. 2005
Improve docstring — add reference to Dworetsky 1983
Add LEMONdB.get_instrumental_magnitudes()
Add unit test for LEMONdB.get_instrumental_magnitudes()
Add instrumental magnitude checkbox
Add 'dump_instrumental_magnitude' to DEFAULT_CURVEDUMP_OPTS
Read initial value of the checkbox from the configuration file
Write value of the checkbox to the configuration file when toggled
Update ExportCurveDialog.__init__() — take a LEMONdB handle
Update ExportCurveDialog.dump() — export instrumental magnitudes
Merge branch 'export-magnitudes' into update-export
Add instrumental S/N checkbox
Add 'dump_instrumental_snr' to DEFAULT_CURVEDUMP_OPTS
Read initial value of the checkbox from the configuration file
Write value of the checkbox to the configuration file when toggled
Update ExportCurveDialog.dump() — export instrumental S/N
Merge branch 'export-SNRs' into update-export
Merge branch 'update-export' into export-instrumental-magnitudes
Give the option to export instrumental magnitudes and SNRs to a file

As requested by Scott Engle in #38, add two checkboxes to the 'Export
to text file' dialog to allow the user to write to the output file the
instrumental magnitudes and signal-to-noise ratios of the astronomical
object, as opposed to the differential magnitudes and SNRs. Among many
other things, they can be used to compute color indices.

This merges branch 'export-instrumental-magnitudes'