Add some common helper functions

[kkappler]

• these functions were in aurora, but depend solely on mt_metadata
• make filters for handling data in V/m so they are treated as mV/km, and data in T so calibrate to nT

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (4922652) 83.90% compared to head (bd09a70) 83.96%.

Files	Patch %	Lines
mt_metadata/timeseries/filters/helper_functions.py	91.89%	3 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #185 +/- ## ========================================== + Coverage 83.90% 83.96% +0.05% ========================================== Files 269 269 Lines 20045 20094 +49 ========================================== + Hits 16819 16871 +52 + Misses 3226 3223 -3 ``` | [Flag](https://app.codecov.io/gh/kujaku11/mt_metadata/pull/185/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JP) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/kujaku11/mt_metadata/pull/185/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JP) | `83.96% <94.44%> (+0.05%)` | :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=JP#carryforward-flags-in-the-pull-request-comment) to find out more.

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