GRINS_detection_from_BOWTIE without interactive display

[surh]

As noted in issue #21

Systems that don't support interactive display needed for plots created by the GRINS_detection_from_BOWTIE.py script finish the script with an error.

Solution from issue #21 is to deactivate the default Matplotlib Xwindows backend with:

import matplotlib
matplotlib.use('Agg')

This needs testing, but should probably be made the default.