docs(examples): use pooch for data access in tutorials/examples

[wpbonelli]

First step towards #1872. This is ugly, but it makes notebooks runnable (provided exes and python environment) out of the box. Local files will be used if detected, otherwise downloaded, following the pattern in the mf6 example models.

An eventual models API could hide all the details of model access.

Also mention the optional dependencies requirement on the tutorials and examples gallery pages.

• feat_working_stack_examples.py
• mf6_output_tutorial01.py
• mf6_parallel_model_splitting_example.py
• mf6_sfr_tutorial01.py
• modpath6_example.py
• mt3dms_examples.py
• nwt_option_blocks_tutorial.py
• plot_array_example.py
• plot_cross_section_example.py
• plot_map_view_example.py
• raster_intersection_example.py
• modflow_postprocessing_example.py
• modelgrid_examples.py
• mfusg_conduit_examples.py
• mf6_complex_model_example.py
• mf_load_tutorial.py
• mf_error_tutorial.py
• load_swr_binary_data_example.py
• groundwater2023_watershed_example.py
• groundwater_paper_uspb_example.py
• export_vtk_tutorial.py
• export_tutorial.py
• array_output_tutorial.py
• zonebudget_example.py
• vtk_pathlines_example.py
• uzf_example.py
• shapefile_feature_examples.py
• shapefile_export_example.py
• sfrpackage_example.py
• mf6_mnw2_tutorial01.py
• mt3dms_sft_lkt_uzt_tutorial.py
• mf6_support_example.py
• mfusg_freyberg_example.py
• mt3d-usgs_example.py

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.9%. Comparing base (bb9824e) to head (5bf2a4d).
Report is 16 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2392     +/-   ##
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+ Coverage     68.4%   75.9%   +7.5%     
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  Files          294     295      +1     
  Lines        59390   61665   +2275     
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+ Hits         40652   46857   +6205     
+ Misses       18738   14808   -3930     

see 229 files with indirect coverage changes