feat: move PSP to coordinates module

[nstarman]

This PR moves PSP from dynamics to coordinates for 2 reasons:

1. conceptual: PSP is a coordinate.
2. submodule dependencies to prevent circular imports. This is especially important with runtime type-checking where we can't use if TYPE_CHECKING blocks the same way as non-runtime-type-checked code. We want the following hierarchy (where higher items can depend on lower items):
   utils -> coordinates -> integrate / potential -> dynamics.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1e1f081) 87.71% compared to head (77e6eab) 87.78%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   87.71%   87.78%   +0.06%     
==========================================
  Files          42       43       +1     
  Lines        1254     1261       +7     
==========================================
+ Hits         1100     1107       +7     
  Misses        154      154              

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