Added optional flag to time_interp_external to speedup initialization #459

Job	Run time
build (-DOPENMP=on)	8m 28s
build (-DOPENMP=on, -DUSE_DEPRECATED_IO=on)	8m 57s
build (-DOPENMP=on, -DWITH_YAML=on)	9m 50s
build (-DOPENMP=on, -DWITH_YAML=on, -DUSE_DEPRECATED_IO=on)	9m 7s
build (-DOPENMP=off)	7m 55s
build (-DOPENMP=off, -DUSE_DEPRECATED_IO=on)	8m 37s
build (-DOPENMP=off, -DWITH_YAML=on)	8m 4s
build (-DOPENMP=off, -DWITH_YAML=on, -DUSE_DEPRECATED_IO=on)	8m 42s
	1h 9m 40s

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