Test for integer axes creation

[yallup]

Description

Introduces a make_Nd_axes test for integer axis creation with a few different patterns

Addresses request raised in #321

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 anesthetic tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works
• I have appropriately incremented the semantic version number in both README.rst and anesthetic/_version.py

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (d3aafc2) 100.00% compared to head (d16a80c) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #322   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        33           
  Lines         2790      2792    +2     
=========================================
+ Hits          2790      2792    +2     

Files Changed	Coverage Δ
anesthetic/_version.py	100.00% <100.00%> (ø)
anesthetic/plot.py	100.00% <100.00%> (ø)

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[williamjameshandley]

OK, I think that squashes this bug, and now is more robust to mixed indices. If you approve, please squash and merge @yallup