added default rasterization to scatter_plot_2d()

[mrosep]

Description

Hello all! Have created a new pull request for rasterizing scatter plots (and maybe other plot kinds too) as increasingly they take a long time to load on certain pdf viewers. This is an old pr but I accidentally closed the other one (lol). The rasterize setting is now embedded deeper in the code (in anesthetic/plot.py in the scatter_plot_2d() function) and rasterizing scatters has been made the default. I've also done away with z_orders as they were a bit awkward and I don't think we need them if we just use the matplotlib keyword argument 'rasterized'? Does anyone see an issue with doing it this way? Let me know your thoughts!

Fixes # (issue)

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

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (79c7bb6) to head (3892b52).

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #380      +/-   ##
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- Coverage   100.00%   99.67%   -0.33%     
===========================================
  Files           36       36              
  Lines         3052     3053       +1     
===========================================
- Hits          3052     3043       -9     
- Misses           0       10      +10     

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