fix issue with appointment length computation - #78

Closes #78 (closed)

I compared with the previous version with the multiselect and I didn't see any difference in terms of how the duration is computed. From all appointment types that can be executed in parallel we take the max duration (max_parallel) For all sequential types, we take the sum (sum_sequential) the total time is max(sum_sequential, max_parallel), i.e. we consider that the sequential tasks are independent from the parallels tasks.