NWTClib: call GetNewUnit in every file open command #1174

Job	Run time
build-all-debug-single	1m 32s
build-all-debug	2m 6s
build-drivers-release	3m 22s
build-postlib-release	3m 27s
build-test-uadriver-debug	1m 35s
rtest-modules-debug	0s
rtest-module-drivers	0s
build-openfast-release	0s
build-fastfarm-release	0s
build-interfaces-release	0s
rtest-OF-5MW_OC3Mnpl_DLL_WTurb_WavesIrr	0s
rtest-OF-5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth	0s
rtest-OF	0s
rtest-OF-5MW_OC4Semi_WSt_WavesWN	0s
rtest-FF	0s
rtest-OF-5MW_Land_BD_DLL_WTurb	0s
rtest-OF-5MW_OC3Trpd_DLL_WSt_WavesReg	0s
rtest-OF-linearization	0s
rtest-interfaces	0s
	12m 2s

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