Add P3 terminal velocity functions

[trontrytel]

This PR:

• Adds the rain and ice individual particle terminal velocities needed for P3 scheme numerical integrals. A peel off from Add P3 Sink Terms #381.
• Fixes a typo in one of the velocity difference computations.
• Removes the TerminalVelocity file and adds the necessary functions in the Microphysics2M (for rain) or P3_terminal_velocity (for ice) files.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.73%. Comparing base (9cf787d) to head (01ca473).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
- Coverage   96.99%   96.73%   -0.27%     
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  Files          41       40       -1     
  Lines        1465     1469       +4     
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  Hits         1421     1421              
- Misses         44       48       +4     

Components	Coverage Δ
src	98.61% <77.77%> (-0.29%)	⬇️
ext	69.79% <ø> (ø)

[rorlija1]

I'm not sure if there are rules as to what to comment on in a PR review: But I appreciate the TODOs you added, and I noticed that since the velocity difference function will be used only when we merge the processes, it should be good to merge it even though it's not covered by the tests. Looks great!