Add unittest for gridded timeseries

[mhidas]

Fixes #94 ... and fix encoding of fill values in gridded product (was getting NaNs before, despite having a _FillValue attribute set)

[codecov[bot]]

Codecov Report

Base: 85.54% // Head: 95.41% // Increases project coverage by +9.86% :tada:

Coverage data is based on head (e713ce5) compared to base (2b1f81e). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #177 +/- ## ========================================== + Coverage 85.54% 95.41% +9.86% ========================================== Files 10 10 Lines 1128 1135 +7 Branches 156 154 -2 ========================================== + Hits 965 1083 +118 + Misses 144 28 -116 - Partials 19 24 +5 ``` | [Impacted Files](https://codecov.io/gh/aodn/python-aodntools/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aodn) | Coverage Δ | | |---|---|---| | [...odntools/timeseries\_products/gridded\_timeseries.py](https://codecov.io/gh/aodn/python-aodntools/pull/177?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aodn#diff-YW9kbnRvb2xzL3RpbWVzZXJpZXNfcHJvZHVjdHMvZ3JpZGRlZF90aW1lc2VyaWVzLnB5) | `90.07% <100.00%> (+90.07%)` | :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=aodn). 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=aodn)

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[mhidas]

Thanks @leonardolaiolo ! Feel free to add your time spent here: https://github.com/aodn/PO-Backlog/issues/897