Discrepancies in examples

[dandavies99]

Related to JOSS review

The examples are excellent - really clear and easy to follow. I encountered just a few problems:

• mu_vs_mu.calculate() returns a tuple which breaks Surfaces Tutorials 1, 2 and 3 (and the docstring needs updating accordingly).

• I seem to get a different answer for bulk/Tutorial_4 compared to the plots that are pre-loaded in the notebook. See below for the first example. I'm not sure what the source of the difference could be but might be something you'd like to check.

• Similar situation for the last plot of bulk/Tutorial_6

[jstse]

Updated tutorail pictures and should now match the plots you recieved. Issue was old pictures in tutorials not being updated when changing plotting values for examples. See pull request #27