Misc fixes from issues

• Fixed bug when calculating deadlines for algorithms where the ALAP schedule contained laps.

• Replaced all startswith checks with isinstance for operations.

• Updated SchedulingGUI to show "∞" instead of maximum integer for forward and backward slack.

• Added enforcing of execution time in ListScheduler and added test to check that latency-offsets and execution times will raise Exceptions if missing.

• Updated ALAP scheduler to adjust scheduling time if a "gap" appears in the start of the schedule.

Closes: #324 (closed) #322 (closed) #318 (closed) #277 (closed) #327 (closed)