fix(prt): make extended tracking opt-in

[wpbonelli]

Previously the PRT model's default behavior was to track particles until termination, as with MODPATH 7 stop time option 2 (extend). Under extended tracking, the program may not halt if particles enter a cycle in the flow system. This PR changes the default to the equivalent of MP7 stop time option 1 (final), terminating at simulation end unless a new Particle Release Point (PRP) package keyword option EXTEND is provided. This is meant to provide a stronger guarantee that the program halts under default settings.

The extend option is stored as an integer rather than logical in case we need to enumerate more alternatives in future.

This PR also avoids unnecessary iterations in MethodSubcellPollock/Ternary, and changes the default visibility of ParticleStore members to private, as per style conventions.

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Checklist of items for pull request

• Added new test or modified an existing test
• Ran black on new and modified autotests
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated definition files
• Updated develop.tex with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Updated input and output guide
• Removed checklist items not relevant to this pull request

[langevin-usgs]

@wpbonelli, thanks for doing this. I think this will save some trouble. One thing that that jumps out at me is if there is a clearer keyword to use for this option. Certainly "EXTEND" is fine and is clearly described. Maybe "EXTEND_TRACKING" or something with TIME in it?

Also wonder if it would be worth explicitly mentioning that if the extend option is specified that particles will continue to be tracked using the flows from the last time step. This is probably obvious but I think it's good to mention. For a transient simulation with a changing flow field, flows from the last time step may not be a good indicator of conditions past the end of the simulation.

[wpbonelli]

Thanks @langevin-usgs, renamed EXTEND -> EXTEND_TRACKING and mentioned that the flows from the final time step are used for extended tracking.