Added ion scaling tune shift tests

[ewaagaard]

Added tests of maximum tune shift from space charge in the SPS. Compares maximum tune shift of different ions with Q^2/A = 1 vs protons.

Closes #251 in xsuite

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts