Added functions to find direct connections from a station

Added functions to find the direct fixed links from a given station
on a given day and time and the direct scheduled services. A
timetable should be extracted into a temporary table using
mca.get_full_timetable for the latter.
Add iterate_reachable to find all stations (indirectly) reachable

Given a starting time and location, the function iterate_reachable
iterates over a list of stations and arrival times, adding any further
stations that can be reached from the existing stations and the
arrival times at those stations. After several iterations, the fastest
path to every station on the rail network will be calculated.