fix(gwf3chd8.f90): account for auxiliary multiplier value in IDM modifications to CHD #1293

Job	Run time
Check format	1m 46s
Build	2m 34s
Smoke test	5m 29s
Parallel testing (ubuntu-22.04)	9m 11s
Test intel fortran (ubuntu-22.04, intel-classic, 2021.7)	22m 57s
Test gnu fortran (ubuntu-22.04)	33m 51s
Parallel testing (macos-12)	20m 37s
Test gnu fortran (macos-12)	39m 22s
Test intel fortran (macos-12, intel-classic, 2021.7)	39m 59s
Test gnu fortran (windows-2022)	51m 33s
Test intel fortran (windows-2022, intel-classic, 2021.7)	1h 0m 21s
	4h 47m 40s

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