Improved typing

[codecov[bot]]

Codecov Report

Base: 87.93% // Head: 87.93% // No change to project coverage :thumbsup:

Coverage data is based on head (dbdd583) compared to base (5a279a4). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #335 +/- ## ======================================= Coverage 87.93% 87.93% ======================================= Files 69 69 Lines 10333 10333 ======================================= Hits 9086 9086 Misses 1247 1247 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `87.93% <100.00%> (ø)` | | 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/335?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/335/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL2dyaWRzL2Jhc2UucHk=) | `86.31% <100.00%> (ø)` | | | [pde/tools/math.py](https://codecov.io/gh/zwicker-group/py-pde/pull/335/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL3Rvb2xzL21hdGgucHk=) | `97.46% <100.00%> (ø)` | | | [pde/tools/plotting.py](https://codecov.io/gh/zwicker-group/py-pde/pull/335/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL3Rvb2xzL3Bsb3R0aW5nLnB5) | `70.06% <100.00%> (ø)` | | 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)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.