fix(prt): fix fpe by constraining local z coord to unit interval

[wpbonelli]

In the generalized particle tracking method, a local Z coordinate could be calculated to fall slightly outside of the unit interval due to numerical imprecision. This could cause the vertical travel time calculation to fail with a floating point exception. Constrain the local Z coordinate to the unit interval to prevent this.

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Checklist of items for pull request

• Replaced section above with description of pull request
• Closes PRT error: floating divide by zero in subcell ternary method #2179
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated develop.tex with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Removed checklist items not relevant to this pull request