Modify impact_parameter() to work with N-D arrays

[pheuer]

The impact_parameter function in collisions.py can currently handle a 1D array of densities or temperatures, but not arrays of higher dimension. This PR changes line that handles 1D arrays to work for any shape array.

impact_parameter is called by many of the other functions in the collisions module (for example, `collision_frequency'), so this change will also enable support for higher dimension arrays in some of those functions.

[namurphy]

Thank you, @pheuer! This looks really helpful.

Would you be able to add some tests of this capability? If this function gets revised in the future, I can imagine that this change could get accidentally/unknowingly reverted.

Thank you again!

[codecov]

Codecov Report

Merging #1604 (72f0309) into main (eb4b66b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1604   +/-   ##
=======================================
  Coverage   97.21%   97.21%           
=======================================
  Files          83       83           
  Lines        7961     7961           
=======================================
  Hits         7739     7739           
  Misses        222      222           

Impacted Files	Coverage Δ
plasmapy/formulary/collisions.py	96.96% <100.00%> (ø)
plasmapy/utils/units_helpers.py
plasmapy/utils/units_definitions.py
plasmapy/utils/_units_helpers.py	100.00% <0.00%> (ø)
plasmapy/utils/_units_definitions.py	100.00% <0.00%> (ø)

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[namurphy]

It's looking close! I have a couple of minor suggestions, but we can probably merge this today.

[pheuer]

Those suggestions look good to me - thanks @namurphy !

[namurphy]

Cool! Assuming everything passes, we can merge this just in time for the 0.8.0 release which may happen Friday we hope.

[namurphy]

I'll enable auto-merge since as long as tests pass then this will be ready. Thanks again for doing this!