[bwentl]

Fix for #1133

• expressions.compute_columns now adds network_los to the locals used to evaluate expressions, next to skim_dict, so preprocessor and annotate-table specs can call network_los.skim_time_period_label(...) and read network_los.skim_time_periods.
• This fixes the NameError: name 'network_los' is not defined raised when a trip_mode_choice annotate-trips spec uses network_los.
• Tests: the test_preprocessing preprocessor spec gains a los_period column computed with network_los.skim_time_period_label(df.depart), and the fixtures register network_los and supply a depart column. test_preprocessing.py passes (2 passed).
• No documentation change is needed. The change only makes an existing network_los object available in expression files.