Powf to powi

[drobnyjt]

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Fixes #306

Description

This draft PR replaces all unnecessary powf invocations with appropriate alternatives.

Tests

cargo test has been run; I'll use CI to run integration tests and check output is unchanged.

[drobnyjt]

This PR also fixes some default initial ion positions in the Python API - they were erroneously converted from meters to angstroms with 10e30 instead of 1e30 - this does not have a measurable effect on the vast majority of simulations which is how it had been missed for so long, but if someone were using reflection_coefficient for sub-eV simulations (which is explicitly not recommended) it could have an effect on results