Derive start and finish dates and elapsed durations where possible in…

… the task tree

Update to ifcopenshell.util.date for handling of IfcDuration with pyt…

…hon and import of Planned Durations from P6 .xml exports in blenderBIM

Implement feature to select all products related to a task in blender…

…BIM, and the ifcopenshell.api

Add support for editing sequence relationship attributes in blenderBIM

Implement support of importing calendars from P6

Add support for editing calendar work and exception times and recurri…

…ng work times

Aggregate WorkSchedules to WorkPlans

You can now assign derived quantities from products to cost items

New feature to manipulate cost item quantities

Code review with myoualid for IFC resource module