Maintenance/calendar unit tests

[sablanchard]

general maintenance to cover updates made in #70 that solved #69 that removes valueerrors when only calendar_dates is used

• adds unit tests to cover loading of calendar and calendar_dates data, add conftest file
• removed creation of blank calendar df in san diego integration test as its no longer needed
• removes day param and notation from interpolate_stop_times() to make agnostic to usage of calendar or calendar_dates data data
• fixes print for when calendar_dates_lookup fails to find match
• minor formatting on changes from #70
• updates to readme and docs to make support of calendar or calendar_dates clear
• removes valueerror on read calendar file function to make it similar to read calendar_dates file function in #70
• add valueerror for when both calendar and calendar_dates is missing