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