Add unit tests for integrator implementation(s)

[matt-graham]

Adds some basic unit tests for integrators (currently only LeapfrogIntegrator but should be simple to extend to other integrator types) that checks that a few key invariants are maintained by each integrator step:

• that each step keeps all phase point components finite and with a non-zero step size changes all phase point components;
• that the composition of a step, momentum flip, another step, and then another momentum flip is an identity map (so that the integrator is time-reversible);
• that a step with a zero step size $\epsilon$ does not change any of phase point components;
• that integrating over multiple steps the Hamiltonian of the system remains approximately conserved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.36%. Comparing base (34d4ce2) to head (ef9c2c5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #32   +/-   ##
=======================================
  Coverage   89.36%   89.36%           
=======================================
  Files           7        7           
  Lines         141      141           
=======================================
  Hits          126      126           
  Misses         15       15           

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