in test-app-processor, support local file paths

[magland]

You can do something like this:

dendro test-app-processor --app-dir spike_sorting_analysis --processor create_spike_sorting_analysis --context sample_context.yaml

where sample_context.yaml is:

recording: "https://api.dandiarchive.org/api/assets/6864d7f3-08cf-4ae9-bf47-bf29048b7f7b/download/"
sorting: "/home/magland/src/dendro_apps/test_sorting.nwb"
output: "./test.nh5"
electrical_series_path: "/acquisition/ElectricalSeries"

Note that the recording input is a remote url, whereas the sorting input is an absolute local path.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (93bd04d) 70.91% compared to head (6010a16) 70.83%.

Files	Patch %	Lines
python/dendro/sdk/App.py	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #111 +/- ## ========================================== - Coverage 70.91% 70.83% -0.09% ========================================== Files 72 72 Lines 4312 4317 +5 ========================================== Hits 3058 3058 - Misses 1254 1259 +5 ``` | [Flag](https://app.codecov.io/gh/flatironinstitute/dendro/pull/111/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flatironinstitute) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/flatironinstitute/dendro/pull/111/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flatironinstitute) | `70.83% <0.00%> (-0.09%)` | :arrow_down: | 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=flatironinstitute#carryforward-flags-in-the-pull-request-comment) to find out more.

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