bandaid fix to deal with NWM rainrate being slightly negative in some…

[mdeshotel]

There appears to be an issue with the NWM Retrospective dataset where rain rate values fall very slightly negative. As a band aid fix I've replaced all slightly negative rain rate values with zero when reading the source dataset. 

https://jira.nextgenwaterprediction.com/browse/NGWPC-10972

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Testing

1. Testing performed in RTE. The issue was replicated then the remedy tested.

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• PR has an informative and human-readable title
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• Visually tested in supported browsers and devices (see checklist below 👇)
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Testing checklist

Target Environment support

• Linux