do not allow zero probs in trip schedule choice #969

Job	Run time
linux-64-py3.10	6m 30s
estimation_mode_test	16m 24s
macOS	5m 2s
develop-docbuild	0s
prototype_mtc	43m 24s
win-64	10m 8s
placeholder_psrc	37m 21s
random_seed_generation_test	2m 47s
prototype_marin	1m 31s
Standard 1-Zone Example (MTC)	4m 7s
prototype_mtc_extended	1h 15m 11s
Standard 2-Zone Example (SANDAG)	59m 29s
placeholder_sandag	1h 31m 27s
prototype_sandag_xborder	4m 32s
production_semcog	5m 12s
prototype_mwcog	44m 4s
placeholder_multiple_zone	8m 3s
	6h 55m 12s

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