Fix caching of internal projection matrices

[jcapriot]

Change projection caching to depend on both the projection type and the with_volume kwarg.

(And update broken links in the docs. The python-for-scientists site was moved to pyzo.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.29%. Comparing base (0926391) to head (730287c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #375   +/-   ##
=======================================
  Coverage   86.28%   86.29%           
=======================================
  Files          89       89           
  Lines       18716    18726   +10     
  Branches     2961     2961           
=======================================
+ Hits        16149    16159   +10     
  Misses       1881     1881           
  Partials      686      686           

Flag	Coverage Δ
	86.29% <100.00%> (?)

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