Fix unreachable code in projections/isea.

[direvus]

The internal helper function isea_hex() had an early unconditional return, which made most of the code in the function totally unreachable.

Based on the comments in the unreachable section, I speculate that the original author meant for this early return to occur for aperture values other than 3 or 4.

In passing, change the function's return type to void, since it was always returning 1 anyway and the return was not used (per #947), and clean up some rogue whitespace.

Fixes #423.

• [x] Closes #423
• [x] Fully documented, including updating docs/source/*.rst for new API

[rouault]

it would be nice to have test/gie/builtins.gie extended to test the fixed cases

[rouault]

it would be nice to have test/gie/builtins.gie extended to test the fixed cases

this point remains to be done

[direvus]

it would be nice to have test/gie/builtins.gie extended to test the fixed cases

this point remains to be done

I have added some tests on my branch that should exercise those code paths. I haven't independently confirmed the actual projection values are correct.

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