Cleaned up `get_boundary_conditions`

[codecov[bot]]

Codecov Report

Base: 87.70% // Head: 87.70% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (478dab0) compared to base (1a58e3c). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #319 +/- ## ======================================= Coverage 87.70% 87.70% ======================================= Files 69 69 Lines 10383 10381 -2 ======================================= - Hits 9106 9105 -1 + Misses 1277 1276 -1 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `87.70% <ø> (+<0.01%)` | :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=zwicker-group#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/zwicker-group/py-pde/pull/319?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group) | Coverage Δ | | |---|---|---| | [pde/grids/base.py](https://codecov.io/gh/zwicker-group/py-pde/pull/319/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL2dyaWRzL2Jhc2UucHk=) | `89.30% <ø> (+0.12%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group)

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