Update `tools` functions to work with n-dimensional fields

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (58f5b4f) 93.56% compared to head (27c6be7) 93.57%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #447 +/- ## ======================================= Coverage 93.56% 93.57% ======================================= Files 28 28 Lines 3016 3020 +4 ======================================= + Hits 2822 2826 +4 Misses 194 194 ``` | [Files Changed](https://app.codecov.io/gh/ubermag/discretisedfield/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ubermag) | Coverage Δ | | |---|---|---| | [discretisedfield/tools/\_\_init\_\_.py](https://app.codecov.io/gh/ubermag/discretisedfield/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ubermag#diff-ZGlzY3JldGlzZWRmaWVsZC90b29scy9fX2luaXRfXy5weQ==) | `100.00% <ø> (ø)` | | | [discretisedfield/tools/tools.py](https://app.codecov.io/gh/ubermag/discretisedfield/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ubermag#diff-ZGlzY3JldGlzZWRmaWVsZC90b29scy90b29scy5weQ==) | `98.97% <100.00%> (+0.02%)` | :arrow_up: |

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