Fix mangroves and wo plugins

[emmaai]

• enforce the use of mangroves extent shapefile
• set nodata=np.nan explicitly for wo float band as esri doesn't agree with the default nan on float type.

Note: nodata issue of gm to be fixed in odc-algo. The virtue here is that every band (variable) should carry nodata as one of attributes no matter what data type it is, not claiming it's a "better" solution.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.24%. Comparing base (f5e80f1) to head (866e938).

Files	Patch %	Lines
odc/stats/plugins/mangroves.py	60.00%	2 Missing :warning:
odc/stats/plugins/wofs.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #132 +/- ## =========================================== + Coverage 78.00% 78.24% +0.23% =========================================== Files 38 38 Lines 3497 3517 +20 =========================================== + Hits 2728 2752 +24 + Misses 769 765 -4 ```

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