Fix utils.plot configuration

[cosimoNigro]

This PR fixes minor inconsistencies in the plotting utilities of the package. The function to plot the electron distribution Blob.plot_n_e did not have a **kwargs argument to specify line width colors, labels etc...

Additionally I have added a matplotlibrc that is automatically loaded, I used the same used for the agnpy paper. With some minor change.

TODO:

• [x] add a test on the new function;
• [x] re-run the notebooks to show the new outlook for the plots.

[codecov[bot]]

Codecov Report

Merging #113 (a3d7407) into master (a122aaf) will increase coverage by 0.67%. The diff coverage is 97.67%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   96.32%   96.99%   +0.67%     
==========================================
  Files          30       30              
  Lines        2203     2233      +30     
==========================================
+ Hits         2122     2166      +44     
+ Misses         81       67      -14     

Flag	Coverage Δ
unittests	96.99% <97.67%> (+0.67%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
agnpy/emission_regions/blob.py	94.23% <66.66%> (+5.94%)	:arrow_up:
agnpy/tests/test_utils.py	100.00% <100.00%> (ø)
agnpy/utils/plot.py	95.83% <100.00%> (+49.16%)	:arrow_up:

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