Stissplice

More bug corrections.

This version corrects a bug that occurred when there is a unique section with only one pixel.

This version of `stissplice` contains the following changes:

- Improved test suite
- Handling of the data quality flags
- Added an option to merge overlap sections using sensitivity
- Changed how the code chooses the wavelength grid to interpolate the merged sections

This is the very first release of `stissplice`. This code splices Echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) instrument. It can be adapted to work with spectra obtained with other instruments as well.

The code is currently in development and accepting suggestions of features to implement, as well as contributions.