refactors: only import mgcv functions and not entire package

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.09%. Comparing base (f5a2eec) to head (a0abf6c). Report is 55 commits behind head on dev.

Files	Patch %	Lines
R/tfb-spline.R	80.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #108 +/- ## ========================================== - Coverage 74.36% 74.09% -0.28% ========================================== Files 28 28 Lines 1771 1957 +186 ========================================== + Hits 1317 1450 +133 - Misses 454 507 +53 ```

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[fabian-s]

thx!