feat: tolerate boundary conditions on inactive cells

[wpbonelli]

Allow boundary conditions to be placed in inactive or pass-through cells, issuing a warning rather than an error. Previously this caused a fatal error in most packages; some were already permissive.

Changes to base types:

• BndExtType — cellid_to_nlist and nodeu_to_nlist: store_error → store_warning
• DisBaseType — read_list: same treatment
• BndType — bnd_fc: skip non-positive node numbers

Changes to boundary packages (add guards for non-positive node numbers):

• gwf-wel, gwf-drn, gwf-riv, gwf-chd, gwf-ghb (also corrects == 0 to <= 0)
• gwt-cnc, gwt-src
• gwe-ctp, gwe-esl
• swf-flw, swf-cdb, swf-zdg (shared by SWF, CHF, and OLF models; swf-chd reuses gwf-chd)

No changes needed (already skipped non-positive node numbers):

• gwf-rch, gwf-evt
• swf-pcp, swf-evp

---

Checklist of items for pull request

• Replaced section above with description of pull request
• Added new test or modified an existing test
• Ran ruff on new and modified python scripts in .doc, autotests, doc, distribution, pymake, and utils subdirectories.
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated develop.toml 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