extension for landcover pipeline and plugins

[emmaai]

Major changes:

• generalize datasets fusion over >=2 products
• allow time dimension to be ignored for certain products and bands, e.g., masking yearly dataset with a multi-year product
• allow products and bands to be optional when aligning/matching over datasets
• new plugin for landcover l3/l4 staging results, the logic of algorithm is shown in the image below

Minor changes:

• change the default value of click to be none, for better hierarchy of options overwriting, i.e, command line option > yaml option > default option

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.19048% with 65 lines in your changes missing coverage. Please review.

Project coverage is 77.97%. Comparing base (2f7cd3f) to head (b11bfaa). Report is 1 commits behind head on develop.

Files	Patch %	Lines
odc/stats/tasks.py	51.94%	37 Missing :warning:
odc/stats/plugins/lc_veg_class_a1.py	74.66%	19 Missing :warning:
odc/stats/model.py	22.22%	7 Missing :warning:
odc/stats/utils.py	96.92%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #138 +/- ## =========================================== - Coverage 78.29% 77.97% -0.33% =========================================== Files 38 40 +2 Lines 3525 3728 +203 =========================================== + Hits 2760 2907 +147 - Misses 765 821 +56 ```

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