add fallthrough statement

[k-doering-NOAA]

Concisely (20 words or less) describe the issues

Removes a warning

Please Link issue(s)

resolves #[add issue number number], resolves #[optional second issue number if more than 1 issue addressed.]

What tests have been done? Upload any model input files created for testing in a zip file, if possible.

The call-build-ss3-warnings artifact file should be checked for presence of the implicit fallthrough warning (which should no longer be present):

ss.cpp: In function ‘void write_message(int, int)’:
ss.cpp:160:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
  160 |         exitflag = 1;
      |         ~~~~~~~~~^~~
ss.cpp:161:7: note: here
  161 |       case ADJUST:
      |       ^~~~

What tests/review still need to be done? Who can do it, and by when is it needed (ideally)?

@nschindler-noaa , could you check if the fallthrough is intentional, or if this should be break; instead?

Has any new code been documented?

If not, please add documentation before submitting the Pull Request.
-[x] I have documented any new code added (or no new code was added)

is there an input change for users to Stock Synthesis?

• Yes, there was an input change

If so, please provide an example of the new inputs needed.

[New example stock synthesis input goes here]

Check which is true. This PR requires:

• no further changes to r4ss
• no further changes to the manual
• no further changes to SSI (the SS3 GUI)
• no further changes to the stock synthesis change log (new features, bug reports)

Describe any changes in r4ss/SS3 manual/SSI that are needed (if not checked):

If changes are needed in the change log, please fill in the table here:

Action	Topics	Type
[fix, new, or revise]	[e.g., biology. Use issue label options.]	[input, output, and/or calc, or ALL]

Additional information (optional):

[k-doering-NOAA]

I checked and the fallthrough statement is indeed gone! The call-build-ss3-warnings is still failing as it is on main because of other warnings (that would need to be fixed on the admb side of things, and may have been fixed already!)

[nschindler-noaa]

The fallthrough statement is correct.