Alternative method to iterating over field components

[JamesMcClung]

This isn't necessary for anything at all, it's essentially an aesthetic refactor of the field_to_component logic. I (currently) prefer it, but it's no big deal to reject this PR.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pscpy/psc.py	92.30%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/pscpy/psc.py	95.83% <92.30%> (-2.42%)	⬇️

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[JamesMcClung]

The failing code coverage is a symptom of a lack of sample data for tests. The fact that this fails is a point in the PR's favor, actually. If I had done #26 after this PR, it would have failed because the code coverage bot would have seen that the 1-line diff wasn't actually being tested.