Simplify code by using `Flow` methods `__len__` and `__getitem__`

[janosh]

Saves accessing flow.jobs a bunch.

[codecov[bot]]

Codecov Report

Merging #467 (7c66385) into main (85aabc5) will not change coverage. Report is 1 commits behind head on main. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #467 +/- ## ======================================= Coverage 99.86% 99.86% ======================================= Files 20 20 Lines 1516 1516 Branches 419 419 ======================================= Hits 1514 1514 Misses 2 2 ``` | [Files](https://app.codecov.io/gh/materialsproject/jobflow/pull/467?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=materialsproject) | Coverage Δ | | |---|---|---| | [src/jobflow/core/flow.py](https://app.codecov.io/gh/materialsproject/jobflow/pull/467?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=materialsproject#diff-c3JjL2pvYmZsb3cvY29yZS9mbG93LnB5) | `100.00% <100.00%> (ø)` | | | [src/jobflow/utils/graph.py](https://app.codecov.io/gh/materialsproject/jobflow/pull/467?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=materialsproject#diff-c3JjL2pvYmZsb3cvdXRpbHMvZ3JhcGgucHk=) | `100.00% <100.00%> (ø)` | |

[utf]

Hi @janosh, please could you revert this change, since it overlaps with the __getitem__ stuff proposed in #450. The __len__ part should still be fine.

At some point, we will need to rewrite the __getitem__ stuff in Flow and so with this PR we'd have to rewrite all the tests too.

[janosh]

could you specify what you mean by "have to rewrite the Flow.getitem"? do you want to make it behave like a dict rather than a list of jobs?