Fix fancy indexing for electrical series with Zarr backend

[alejoe91]

Turns out fancy indexing is not supported for zarr.Dataset objects and this was causing an issue in displaying timeseries correctly.

This small PR fixes it by changing the behavior of the timeseries.py and:

• use direct slicing when possible (consecutive indices)
• slice first in time and then in channels otherwise

Not that for NWBwidgets in all cases we can slice directly, because of the channel slider.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 66.24%. Comparing base (fdc0309) to head (03a598a).

Files	Patch %	Lines
nwbwidgets/timeseries.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #283 +/- ## ======================================= Coverage 66.24% 66.24% ======================================= Files 35 35 Lines 3478 3481 +3 ======================================= + Hits 2304 2306 +2 - Misses 1174 1175 +1 ``` | [Flag](https://app.codecov.io/gh/NeurodataWithoutBorders/nwbwidgets/pull/283/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/NeurodataWithoutBorders/nwbwidgets/pull/283/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `66.24% <80.00%> (+<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=NeurodataWithoutBorders#carryforward-flags-in-the-pull-request-comment) to find out more.

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[bendichter]

@alejoe91 it looks like Zarr does actually support fancy indexing, though the syntax is different from numpy

https://zarr.readthedocs.io/en/stable/tutorial.html#advanced-indexing

[alejoe91]

@bendichter should we use the Zarr fancy indexing functions when the dataset is a Zarr object then?

[bendichter]

Yes that would be an immediate fix. But this issue is going to come up for anything downstream of pynwb so maybe it makes sense to fix on the pynwb level. Maybe we could wrap all arrays in xarray?

[bendichter]

@alejoe91 can you also please add checks that this data indexing utility does indeed solve the problem for Zarr dataset objects?