Doc test data instructions

[sphamba]

Summary

• add instructions to clone test-data
• add instructions to contribute to test-data
• remove gpm.tests from automatically generated API documentation
• remove notebooks copied in doc source
• skip plot tests if test-data repository is not cloned

Fixed

• missing __init__.py in gpm/dataset to be recognized as subpackage

[sphamba]

I added some code snippets to the submodule instructions

[sphamba]

I made the test_visualization tests skip if the submodule is not cloned