Running MPCPy with plots in docker without display

[tsnouidui]

This issue is to address an issue which appears when running MPCPy in a docker container.
When MPCPy tries to validate some parameter estimation runs, it uses matplotlib to create plots.
This step requires a display which is by default not enabled in docker. A solution is to switch the backend of matplotlib so it doesn't need a display.

[dhblum]

As proposed in #142, plt.switch_backend('agg') can be used to keep any plotting in the background and not interactive, which will work in Docker. However, this is not a solution to code in MPCPy since it will affect the user's environment without knowledge.

[dhblum]

A solution to this is to change the backend of matplotlib in the script running inside of a docker before importing MPCPy classes. For example:

import matplotlib
matplotlib.use('Agg')
from mpcpy import models
# continue code ...