Added callable k factor multiplication fnc on power

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.29630% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.66%. Comparing base (eb2488b) to head (611c71e).

Files	Patch %	Lines
src/powerbox/tools.py	96.22%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #56 +/- ## ========================================== + Coverage 90.75% 92.66% +1.90% ========================================== Files 5 5 Lines 411 436 +25 ========================================== + Hits 373 404 +31 + Misses 38 32 -6 ``` | [Flag](https://app.codecov.io/gh/steven-murray/powerbox/pull/56/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Murray) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/steven-murray/powerbox/pull/56/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Murray) | `92.66% <96.29%> (+1.90%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Steven+Murray#carryforward-flags-in-the-pull-request-comment) to find out more.

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