Update compute_core_microbiome.py
Update plot_semivariogram.py
Update compute_core_microbiome.py
Update plot_semivariogram.py
Update compute_core_microbiome.py
Update plot_semivariogram.py

The following two lines were added before the pylab imports:
from matplotlib import use
use('Agg', warn=False)
Otherwise, if user's matplotlib install tries to load up a graphical backend without having a DISPLAY, it shows an error.
Merge pull request #1130 from charudatta-navare/set_Agg_backend

Set agg backend