Improve implementation of boundary conditions

[maltezfaria]

• Uses a recursive function to implement boundary conditions (thanks to @NLebbe)
• Normalise the boundary conditions accepted as input in LevelSetEquation
• Improve the docs of LevelSetMethods

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.33333% with 29 lines in your changes missing coverage. Please review.

Project coverage is 25.64%. Comparing base (cec2eab) to head (3ac9cb5). Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/meshfield.jl	51.35%	18 Missing :warning:
src/meshes.jl	66.66%	7 Missing :warning:
src/levelsetequation.jl	73.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== + Coverage 24.31% 25.64% +1.33% ========================================== Files 11 11 Lines 617 655 +38 ========================================== + Hits 150 168 +18 - Misses 467 487 +20 ```

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