Adds tests showing out-of-range data returning from scipy interpolate…

[MoseleyS]

… and adds an np.clip to the affected plugin to overcome this

This was discovered because the interpolation of the rain falling level can produce values fractionally lower (machine-precision) than the snow falling level if both are zero before the interpolation is done.

Description

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.42%. Comparing base (84a8944) to head (284094d). Report is 13 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2007 +/- ## ========================================== + Coverage 98.39% 98.42% +0.03% ========================================== Files 124 128 +4 Lines 12212 12469 +257 ========================================== + Hits 12016 12273 +257 Misses 196 196 ```

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